v0.28.0 — Plan model hardening

Give work items an honest way to be retired: cancelled (deliberately not doing it) and superseded (replaced by another item), plus a supersedes attribute. Both are terminal and non-achieving. Builds on the consolidated vocabulary from WORK-492.

Update the canonical spec and agent-facing docs so the new work terminals and the pending status are described everywhere authors and agents look.

Close the "is it built / is it available?" gap on specs, and give ADRs an honest "considered and declined" state. Rides on the consolidated vocabulary from WORK-492.

Surface the traceability data structurally so missing PRs and ready-to-flip specs are visible without grepping prose.

Backfill the pr attribute on legacy done work / fixed bug items from git merge-commit history, so plan status rollups are rich from day one rather than skeletal (only ~2 of 177 items carry a PR today).

Make the PR-linkage instruction loud (a line inside an example block has a ~1% hit rate) and document the full lifecycle for humans.

Once cancelled / superseded exist, review the plan corpus for items that should be retired rather than left dangling. The main pool is the 13 pending work items, all sourced from SPEC-008 (unbuilt runes): stat, math, concept, exercise, glossary, objective, prerequisite, quiz, partner, job, album, artist, video. A reviewed, per-item judgment pass — not an automated flip.

Sweep the small set of non-terminal, non-ready work items for entries carrying the wrong status, independent of the new terminals. Human-judgment pass over a bounded list.