P0: Decouple the Review Ledger and the Skills Digest From the Working Tree

[ptr727]

Part of #1311. Priority: now. One pull request, Python plus tests.

Why

Two tracked artifacts are renderings rather than state, and both collide between concurrent branches. canonical_review.py record regenerates reports/canonical-review.md, which carries global counts, so two branches each recording one pass merge cleanly to a wrong count and report --check fails an unrelated later pull request (#1290, #1268). .claude-plugin/fleet-skills/.source-digest is one hash over every skill's bytes, so any two concurrent skill edits conflict (#1240). Both make backlog-burndown's no-shared-file rule unsatisfiable, and both cost a merge or a CI failure that no branch caused.

Scope

• Stop tracking reports/canonical-review.md. Generate it in CI as a job summary or artifact from the JSON ledger, and drop the report --check step from .github/actions/validate/action.yml. The JSON ledger stays tracked, sorted, one entry per unit, since that is real state and it merges.
• Take a lock around cmd_record's read-modify-write of the ledger, the same shape local_review.py already uses (canonical_review.py record Has No Lock, So Two Concurrent Records Lose One Pass #1151).
• Replace the single .source-digest with a per-skill digest, or a digest file per skill directory, so two edits to two skills touch two files.
• Update local-strict-review "The Carried-Content Pass" ordering, which currently commits the ledger before the diff pass because the report is tracked. With only the JSON tracked the ordering stays but the wording that names the .md goes.

Rules that bind this task

• A rendering is never committed. State that once in scripts/README.md, where the scripts are described, and nowhere else.
• Every changed gate gets a test that watches it fail: a test that two records applied in either order produce the same ledger, and a test that two skill edits produce disjoint digest changes.
• Read spec/files.json before touching the digest, since the plugin tree is a declared verbatim tree.

Acceptance

• Two branches each recording a pass over a different unit merge into develop with no conflict and CI green.
• scripts/tests/test_canonical_review.py and test_build_dist.py carry the two new cases.

Closes #1268, #1290, #1151. Relates #1240, #1295.