freshness-and-drift-detection: read surfaces — surface git-derived staleness on find/search and MCP results

[tcballard]

Sub-issue of #277 (freshness-and-drift-detection, ADR-093).

Corpus: rac/requirements/rac-freshness-read-surfaces.md

Scope:

• rac find/search output and MCP search_artifacts results gain last-committed recency + a staleness indicator, joining the provenance get_artifact already embeds — additive (ADR-007), derived solely from the git-recency service (ADR-045), no frontmatter field.
• Degrades to null/absent outside git, for untracked files, and on shallow history — never an error, never a fabricated date.
• Indicator = documented deterministic function of age vs an explicit threshold (configurable, stated default), reported as data beside its date (ADR-034).
• No matching/ordering change in this phase: ADR-078 ranking contract and goldens untouched.
• Golden stability: fixtures pin git state or byte-pinned goldens exclude the fields; MCP budget holds (ADR-033).

Tick on #277 when landed.