* fix(#2989): anchor code-review diff-base grep to phase-mention convention
The diff-base fallback in code-review.md used git log --grep with a bare
phase number (unanchored substring), matching version strings, dates, issue
refs, and other phases' numbers. tail -1 took the oldest match — routinely
a commit from months or years before the phase existed. The fail-closed
branch was dead code because a bare digit almost always matches something.
Changed --grep to '[Pp]hase N\b' with --extended-regexp, anchoring to the
phase-mention convention. When no commit genuinely references the phase,
the derivation yields empty and the fail-closed warning fires (now
reachable). All three consumers (Tier 3 file scope, fallow pre-pass, agent
context) use the same corrected value.
* chore(#2989): backfill changeset PR number 3096
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Co-authored-by: sim <sim@local>
2a77e50daf