fix(upgrade): regenerate built-in SKILL.md when bundled template drifts (PROP-337 v1)

[JKHeadley]

Summary

Closes Portal-tracked PROP-337 — chronic for 21 cycles. The non-destructive migrateBuiltinSkills only writes SKILL.md files that don't already exist; when the inline template in installBuiltinSkills is updated in a new release, existing installs never see the new content. The fix lives only in fresh installs, which is exactly the drift gap PROP-337 names.

This adds BuiltinSkillRegenerator and a new migrateBuiltinSkillRegeneration step in PostUpdateMigrator that:

• Renders the bundled template into a tempdir by invoking installBuiltinSkills (no refactor of the 1187-line function needed).
• Records the SHA-256 of each installed skill in .instar/state/builtin-skill-fingerprints.json.
• On each migrator run, compares on-disk hash against fingerprint+bundled hash:
  • missing → install
  • on-disk == bundled → in sync (refresh fingerprint)
  • on-disk == fp → safe regenerate, update fingerprint
  • else → user-customized, preserve, seed fp on first observe
• Custom skills are never enumerated, so they're untouchable.

Hooks already overwrite unconditionally (verified) so they're out of scope for this PROP — the gap was SKILL.md drift only.

Test plan

• 7 new unit tests cover install, idempotency, regenerate-on-template-bump, preserve-user-customizations, first-observe seeding, and dry-run.
• All 71 existing PostUpdateMigrator tests still pass.
• Pre-push smoke tier passes in CI.
• Manual verification on next release: existing install picks up an updated bundled SKILL.md without losing user customizations.

Cross-repo close-out

Portal-side upstream-prop-watcher is registered against this branch (commit 33c0c353, file src/core/BuiltinSkillRegenerator.ts). Once this PR merges to main, the next cycle-escalator run auto-flips PROP-337 to COMPLETED in Portal's evolution queue — closing the cross-repo loop the chronic detector kept reopening.

🤖 Generated with Claude Code