docs: add Phase 2 quality ledger implementation plan (#886)

Author: ckenst

PHASE_2_PLAN.md

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+## Phase 2 Plan: Quality Ledger & Metrics
+
+### Summary
+- Implement Phase 2 on branch `codex/phase-2-quality-ledger`.
+- Capture one quality snapshot per release tag (`vNNNN`) with Lighthouse, broken-link count, and build time.
+- Persist snapshots in-repo for future dashboarding, while preventing metrics commits from triggering deploy/version loops.
+- Add an issue template field for `Site Version` so bug reports can be attributed to releases.
+
+### Implementation Changes
+- Update deploy workflow in `.github/workflows/deploy.yml`:
+  - Move tag creation to after the Pages deploy step so a tag represents an already-deployed version.
+  - Add `paths-ignore` for the quality ledger file(s) so metrics-only commits do not trigger deploy/version bump.
+- Add a new workflow (for example `.github/workflows/quality-ledger.yml`) that runs on tag pushes matching `v*` and on manual dispatch:
+  - Read version from the tag.
+  - Run `bundle exec jekyll build` and record elapsed seconds.
+  - Serve `_site` locally and run Lighthouse audits; capture Performance, Accessibility, and SEO.
+  - Run `linkinator` against the built/served site and capture broken-link count.
+  - Append a new record to `_data/quality_log.yml` with: `version`, `release_date`, `commit_sha`, `lighthouse.performance`, `lighthouse.accessibility`, `lighthouse.seo`, `broken_links`, `build_time_seconds`, `workflow_run_url`.
+  - Commit the updated ledger back to `main` with a bot commit message.
+- Add/enable issue templates:
+  - Create `.github/ISSUE_TEMPLATE/bug_report.yml` with required `site_version` field and helper text to copy from footer (`rev.XXXX`).
+  - Add `.github/ISSUE_TEMPLATE/config.yml` (disable blank issues if desired) and direct bug reporters to include version.
+- Documentation updates:
+  - Update `README.md` with “Quality Ledger” behavior and where the data lives.
+  - Mark Phase 2 checklist progress in `ROADMAP.md` as each item lands.
+
+### Public Interfaces / Data Contract
+- New data contract in `_data/quality_log.yml`: one entry per released version with stable keys listed above.
+- New contributor-facing interface: GitHub bug issue form with required `Site Version`.
+- CI contract: release tags (`vNNNN`) become the trigger for quality snapshot collection.
+
+### Test Plan
+- Workflow validation:
+  - Run `quality-ledger` via `workflow_dispatch` once to verify parsing/output and ledger append format.
+  - Push/create a test tag in a safe branch/repo context to confirm tag-trigger behavior.
+- Data validation:
+  - Ensure new ledger entry is appended (not overwritten) and version key matches tag.
+  - Confirm numeric parsing for Lighthouse scores, broken links, and build seconds.
+- Regression checks:
+  - Confirm metrics-only ledger commit does not trigger deploy workflow.
+  - Confirm normal content changes still trigger deploy and version bump.
+- Issue template checks:
+  - Open “New issue” UI and verify `site_version` is required and guidance text is present.
+
+### Assumptions & Defaults
+- Defaulted storage choice: in-repo ledger at `_data/quality_log.yml` (chosen to support Phase 4 dashboarding).
+- Lighthouse target is the locally served built site for repeatable CI numbers; production URL checks can be added later as a secondary metric.
+- Bug counts are “attributable” in Phase 2 by collecting `Site Version` in issue intake; aggregate reporting is deferred to Phase 4 dashboard/trends.