docs(nix-retirement): closure report + machine-readable record (#102 #103) (#149)

Refs standards#101 standards#102 standards#103.

Companion to the Wave-0 inventory worklist
(`spec/nix-retirement-inventory.adoc`). The wave-time per-repo
`flake.nix` vs `(Guix manifest ∪ sealed-container)` diff is now complete
across Waves 1–14, and this PR adds the closure documentation that the
owner can use to close the campaign issues.

## What lands

- **`rhodium-standard-repositories/spec/nix-retirement-closure.adoc`** —
human-readable closure report: aggregate result, REMOVE table, KEEP+DEP
rule-3 annotations, over-called-REMOVE corrections, per-issue closure
recommendations, scaffold-stub remediation pivot, verification
discipline.
- **`.machine_readable/6a2/NIX-RETIREMENT.a2ml`** — structured record of
the same, suitable for machine consumers / future agents.

## Aggregate

| outcome | count |
|---|---|
| pure-mirror → REMOVE (verified, PR'd) | **1**
(hyperpolymath/hypatia#289) |
| load-bearing → KEEP+DEP (rule-3 annotated, PR'd) | **10** (007#12,
boj-server#83, cyo#3, echidnabot#13, file-soup#37, game-server-admin#16,
project-wharf#30, proof-of-work#61, tma-mark2#26, ubicity#54) |
| scaffold-stub `guix.scm` → KEEP+FLAG | ~270 |
| over-called REMOVE → demoted on verification | 2 (proof-of-work,
tma-mark2) |

## Pivot

The actual remaining estate-wide debt is **scaffold-stub `guix.scm`**,
not Nix-mirror retirement. Recommend a separate tracker covering the
~270 KEEP+FLAG repos (populate / delete-both /
convert-to-`.guix-channel`-only). Do NOT subsume under #102 — different
gate, different ergonomics.

## Closures (owner action)

The closure doc and the `.a2ml` carry a per-issue
closure-recommendations table for #102, #103, and #106–#119. Per
ISSUE-CLOSURE: bots do not self-close — this PR is `Refs`, never
`Closes`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath