feat(engine): purge work-unit scratch cache at close + migration 050 backfill

[leeovery]

Banked from the render-surfaces programme (design log #489, stage-5 notes): the cache-purge-at-close item, now as its own PR.

What

Close purges scratch. workunit complete, workunit cancel, and workunit absorb (the absorbed feature's cache) remove .workflows/.cache/{wu}/ inside the transaction. Caches are rebuildable per-phase scratch — their job is surviving a context refresh mid-phase; reactivation regenerates them on demand, and the restart paths already rm -rf them.

Disk-only, with one honest guard. Cache is gitignored (.workflows/.gitignore, migration 049), so the purge is normally pure disk. But gitignore never untracked files committed before 049 — so purgeWorkUnitCache checks the index and, when tracked residue exists, hands the pathspec to the transaction commit so the deletions land inside it rather than lingering as dirt outside the .workflows/{wu} scope.

Migration 050 backfills existing installs: purges caches of already-completed/cancelled units and orphan cache dirs whose unit no longer exists. In-progress units and unreadable manifests keep their caches — unknown state is live state.

During investigation we also confirmed (correcting an earlier mis-read): the durable per-cycle analysis/review artifacts live in the phase directory (.workflows/{wu}/implementation/{topic}/…), not cache — nothing deliberately commits cache; the split is already right.

Test plan

• Migration 050 suite: happy path (completed+cancelled purged, in-progress kept), orphan purge, malformed-manifest keep, no-cache-root skip, idempotency, live-content byte preservation.
• Engine: complete purges untracked scratch with zero dirt; tracked pre-049 residue deletions ride the completion commit (asserted via git show --name-only); cancel purges; absorb purges the feature's cache.
• npm test 1543/1543 · npm run test:migrations 10/10 · typecheck clean.

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