docs(governance): allow in-place refinement of accepted ADRs

Replace the strict-immutability rule in docs/decisions/README.md ("do not
edit the original") with the mainstream refine-in-place vs. supersede-on-
reversal model, and add the bookkeeping that makes in-place refinement
auditable.

- Distinguish lifecycle state (proposed/accepted/superseded/deprecated)
  from the orthogonal mutability operation (refine vs. supersede).
- Add a boundary test: if the action a reader takes is unchanged it is a
  refinement (edit in place); if a past reader would now act differently it
  is a reversal (new superseding ADR).
- Add a parallel "Changing an accepted ADR" table (Refinement / Reversal /
  Obsolescence).
- Add optional **Last-updated:** field (distinct from immutable **Date:** =
  decision date) and a ## Changelog convention to the template.
- Cite precedent (Joel Parker Henderson; MADR last-updated date).
- Regenerate the Starlight mirror.

Removes the contradiction with ADR-012 ("existing ADRs are updated
incrementally") and unblocks the ADR-003 decomposition (#186).

Refs #380

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: scottschreckengaust

docs/decisions/README.md

@@ -20,6 +20,7 @@ Do **not** write an ADR for routine implementation choices that are self-evident
 
 **Status:** proposed | accepted | superseded | deprecated
 **Date:** YYYY-MM-DD
+**Last-updated:** YYYY-MM-DD (optional; add when the ADR is refined in place after acceptance)
 **Supersedes:** ADR-NNN (if applicable)
 **Superseded by:** ADR-NNN (if applicable)
 
@@ -41,8 +42,16 @@ What follows from this decision:
 ## References
 
 - Links to RFCs, issues, PRs, or external resources that informed the decision
+
+## Changelog
+
+(Optional; present once the ADR has been refined in place. One dated bullet per refinement. The decision date in `**Date:**` is never overwritten.)
+
+- YYYY-MM-DD — What was refined and why (decision unchanged).
 ```
 
+`**Date:**` records when the decision was **made/accepted** and is never overwritten — it is the historical anchor (which other ADRs existed, what the constraints were). `**Last-updated:**` records when the record was last **refined in place**; the `## Changelog` records *what* changed. See [Changing an accepted ADR](#changing-an-accepted-adr-refine-in-place-vs-supersede).
+
 ## Numbering
 
 ADRs are numbered sequentially with zero-padded three-digit prefixes: `ADR-001-slug.md`, `ADR-002-slug.md`, etc. Numbers are never reused.
@@ -56,7 +65,24 @@ ADRs are numbered sequentially with zero-padded three-digit prefixes: `ADR-001-s
 | `superseded` | Replaced by a newer ADR (link to successor) |
 | `deprecated` | No longer applicable (context changed) |
 
-A decision starts as `proposed` during RFC discussion and moves to `accepted` when the implementing PR merges. To change an accepted decision, write a new ADR that supersedes it — do not edit the original.
+A decision starts as `proposed` during RFC discussion and moves to `accepted` when the implementing PR merges.
+
+### Changing an accepted ADR: refine in place vs. supersede
+
+Lifecycle state (above) is **orthogonal** to ordinary upkeep. Two different operations apply to an accepted ADR, and the status table does not decide between them:
+
+- **Refinement** — the decision is unchanged; you are clarifying wording, adding a consequence or risk, extracting operational prose to a guide/skill (see ADR-012), or fixing a reference. **Edit the file in place.** Status stays `accepted`.
+- **Reversal** — the decision itself changes: a different approach is chosen, or what a reader must do changes. **Write a new ADR that supersedes the old one.** The old ADR becomes `superseded`.
+
+**The boundary test:** *If the action a reader would take is unchanged, it's a refinement — edit in place. If a past reader following the old ADR would now do something different, it's a reversal — write a superseding ADR.*
+
+| Change | What it is | What to do | Status effect | Date fields |
+|--------|-----------|------------|---------------|-------------|
+| Clarify wording; add a consequence/risk; extract prose to a guide/skill (per ADR-012); fix a broken reference | **Refinement** | Edit in place; append a dated `## Changelog` entry | none (`accepted`) | `Date:` unchanged; bump `Last-updated:` |
+| Reverse the decision; choose a different approach; change what a reader must do | **Reversal** | New ADR with `**Supersedes:** ADR-NNN`; mark the old `**Superseded by:** ADR-MMM` | both change | new ADR gets its own `Date:` |
+| The decision no longer applies (context evaporated) | **Obsolescence** | Mark `deprecated`; note why | `deprecated` | bump `Last-updated:` |
+
+This replaces strict immutability with the mainstream ADR practice. Joel Parker Henderson's widely-cited guidance notes that *"in theory, immutability is ideal; in practice, mutability has worked better for our teams"* — insert new information into the existing ADR with a date stamp and a note. MADR likewise tracks a last-updated date distinct from the decision date. The **decision date is never overwritten**; refinements are recorded in the `## Changelog` and the `Last-updated` field, keeping the record self-contained (an agent should not need to excavate git history to learn what was refined and when).
 
 ## Relationship to `docs/design/`
 

docs/src/content/docs/decisions/Readme.md

@@ -24,6 +24,7 @@ Do **not** write an ADR for routine implementation choices that are self-evident
 
 **Status:** proposed | accepted | superseded | deprecated
 **Date:** YYYY-MM-DD
+**Last-updated:** YYYY-MM-DD (optional; add when the ADR is refined in place after acceptance)
 **Supersedes:** ADR-NNN (if applicable)
 **Superseded by:** ADR-NNN (if applicable)
 
@@ -45,8 +46,16 @@ What follows from this decision:
 ## References
 
 - Links to RFCs, issues, PRs, or external resources that informed the decision
+
+## Changelog
+
+(Optional; present once the ADR has been refined in place. One dated bullet per refinement. The decision date in `**Date:**` is never overwritten.)
+
+- YYYY-MM-DD — What was refined and why (decision unchanged).
 ```
 
+`**Date:**` records when the decision was **made/accepted** and is never overwritten — it is the historical anchor (which other ADRs existed, what the constraints were). `**Last-updated:**` records when the record was last **refined in place**; the `## Changelog` records *what* changed. See [Changing an accepted ADR](#changing-an-accepted-adr-refine-in-place-vs-supersede).
+
 ## Numbering
 
 ADRs are numbered sequentially with zero-padded three-digit prefixes: `ADR-001-slug.md`, `ADR-002-slug.md`, etc. Numbers are never reused.
@@ -60,7 +69,24 @@ ADRs are numbered sequentially with zero-padded three-digit prefixes: `ADR-001-s
 | `superseded` | Replaced by a newer ADR (link to successor) |
 | `deprecated` | No longer applicable (context changed) |
 
-A decision starts as `proposed` during RFC discussion and moves to `accepted` when the implementing PR merges. To change an accepted decision, write a new ADR that supersedes it — do not edit the original.
+A decision starts as `proposed` during RFC discussion and moves to `accepted` when the implementing PR merges.
+
+### Changing an accepted ADR: refine in place vs. supersede
+
+Lifecycle state (above) is **orthogonal** to ordinary upkeep. Two different operations apply to an accepted ADR, and the status table does not decide between them:
+
+- **Refinement** — the decision is unchanged; you are clarifying wording, adding a consequence or risk, extracting operational prose to a guide/skill (see ADR-012), or fixing a reference. **Edit the file in place.** Status stays `accepted`.
+- **Reversal** — the decision itself changes: a different approach is chosen, or what a reader must do changes. **Write a new ADR that supersedes the old one.** The old ADR becomes `superseded`.
+
+**The boundary test:** *If the action a reader would take is unchanged, it's a refinement — edit in place. If a past reader following the old ADR would now do something different, it's a reversal — write a superseding ADR.*
+
+| Change | What it is | What to do | Status effect | Date fields |
+|--------|-----------|------------|---------------|-------------|
+| Clarify wording; add a consequence/risk; extract prose to a guide/skill (per ADR-012); fix a broken reference | **Refinement** | Edit in place; append a dated `## Changelog` entry | none (`accepted`) | `Date:` unchanged; bump `Last-updated:` |
+| Reverse the decision; choose a different approach; change what a reader must do | **Reversal** | New ADR with `**Supersedes:** ADR-NNN`; mark the old `**Superseded by:** ADR-MMM` | both change | new ADR gets its own `Date:` |
+| The decision no longer applies (context evaporated) | **Obsolescence** | Mark `deprecated`; note why | `deprecated` | bump `Last-updated:` |
+
+This replaces strict immutability with the mainstream ADR practice. Joel Parker Henderson's widely-cited guidance notes that *"in theory, immutability is ideal; in practice, mutability has worked better for our teams"* — insert new information into the existing ADR with a date stamp and a note. MADR likewise tracks a last-updated date distinct from the decision date. The **decision date is never overwritten**; refinements are recorded in the `## Changelog` and the `Last-updated` field, keeping the record self-contained (an agent should not need to excavate git history to learn what was refined and when).
 
 ## Relationship to `docs/design/`