feat(enrichment): flag PRs significantly behind the default branch

[jaso0n0818]

What

A new local REES analyzer, staleBranch, that flags a PR whose head is significantly behind the repo's current
default branch — a staleness risk GitHub's own PR page doesn't summarize as a number (it only shows an
"out-of-date" badge, not how far).

Detection (structured GitHub repo + compare API fields only — a bounded, documented schema)

• Fetches the repo's current default_branch, then compares it against the PR's head via the GitHub compare API.
• Flags when behind_by >= 100 commits.

Why it is bounded / merge-safe

It reads only documented fields from the GitHub repo API (default_branch) and the compare API (behind_by) and
compares a single number against a fixed threshold — never diff, commit, or file content. There is no
text/YAML/code parsing, so there are no ambiguous-syntax edge cases. Fail-safe throughout: a missing token,
missing head SHA, malformed repo slug, a missing/non-numeric behind_by, or either fetch failing all yield no
finding rather than an error (the repo-info fetch failing short-circuits before ever calling compare). Mirrors
the existing blame-link / approval-integrity / ci-check-signals analyzers' structure (same header/fetch
helper shape, same ScanOptions/fail-safe conventions, a pure reducer separated from the network entrypoint).

Value

A branch far behind the current default branch is more likely to hide a subtle semantic conflict that a clean
mergeable check alone would miss — the merge itself can be mechanically clean while the combined result is
still wrong. Surfacing how far behind (not just "out of date") gives a reviewer a concrete signal for whether a
rebase is worth requesting before merge.

Tests

review-enrichment/test/stale-branch.test.ts covers: the pure evaluateStaleBranch reduction (the threshold's
boundary in both directions, the identical/ahead cases, and fail-closed on a missing/non-numeric/negative
behind_by), plus the network entrypoint's fail-safe paths (no token, no head SHA, malformed repo slug, the
repo-info fetch failing without ever calling compare, a repo response missing default_branch, the compare fetch
failing) and the exact two-request URL shape. Analyzer metadata is regenerated and committed.

No linked issue

No linked issue because this is a net-new analyzer; there is no tracking issue to link.

[loopover-orb]

Important

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed · 🟪 Reviewing}

[JSONbored]

Not something we need, closing.