observability: inject explicit context into canonical review and commit-scroll seam

[ToadAid]

• move review observability onto an explicit commitScroll -> reviewDraftForCanon seam
• let reviewDraftForCanon accept a small observability interface instead of relying on process-global helpers directly
• preserve review status derivation, emitted event names, conflict metric names, validation/review ordering, and result shapes
• keep fallback resolution for other direct callers so non-canonical paths continue to work
• verification passed:
  • pnpm check
  • pnpm vitest run src/mirror-review/review_engine.test.ts src/mirror/skills/commit_scroll/commit_scroll.test.ts src/mirror-observability/observability.test.ts

[ToadAid]

• move review observability onto an explicit commitScroll -> reviewDraftForCanon seam
• let reviewDraftForCanon accept a small observability interface instead of relying on process-global helpers directly
• preserve review status derivation, emitted event names, conflict metric names, validation/review ordering, and result shapes
• keep fallback resolution for other direct callers so non-canonical paths continue to work
• verification passed:
  • pnpm check
  • pnpm vitest run src/mirror-review/review_engine.test.ts src/mirror/skills/commit_scroll/commit_scroll.test.ts src/mirror-observability/observability.test.ts