Seal hash simplification

Stale commit assertion passes trivially: `artifact.test.ts:1233,1242` — the "appends to existing .saves.json" test saves `scope.commit` and later asserts it's unchanged. Since `writeSaveMetadata` no longer writes `commit`, both values are `undefined`, so `expect(undefined).toBe(undefined)` passes without testing anything. The test still verifies scope entry survives a spec save via `toBeDefined()` at line 1239, but the commit-specific line is dead weight. Next cycle touching this test should remove lines 1233 and 1242.

`execSync` import retained but usage reduced: `verify.ts:17` — `execSync` is still imported and used for the merge-base/diff commands (lines 150-157). The old `git show` call was the fragile one (required exact commit hash). The remaining `execSync` calls use merge-base, which is robust. Not a problem — just noting that the file still has a child-process dependency for the scoped search path.

A005 fallback test triggers via missing ana.json, not missing branch: `verify.test.ts:548-590` — the fallback test causes `readArtifactBranch` to throw by omitting `ana.json` entirely. This is a valid trigger but tests a different failure mode than "artifact branch exists but merge-base fails" (e.g., on a repo with no shared history). Both paths hit the same catch block, so the behavioral coverage is equivalent. A future cycle could add a test where `ana.json` exists but `git merge-base` fails (orphan branches, shallow clones).

pr.test.ts fixture retains old `commit` fields: `pr.test.ts:278` — the fixture has `commit: 'def456'` on the contract entry. This represents old-format data and is harmless (JSON.parse ignores extra properties), but it means the pr.test.ts fixture doesn't reflect what current code produces. If a future test asserts on the shape of `.saves.json` entries, this fixture would give a false impression. Low priority.