fix(worker): make Jira repo selection smart via code evidence - #42

Summary

TICKET-7881 ("override Tailwind tracking-tighter token") mis-routed to fitness-club-daily-schedule instead of web-app. Root cause: select_repo fed the LLM only each repo's name / description / top-level dirs and asked it to guess — a generic ticket has no repo hint in its prose, so the pick was arbitrary. (A hard CPM→web-app mapping was rejected as a bandaid; the selector itself needs to be smart.)

Fix — ground selection in code evidence

The repo that actually contains the ticket's identifiers is almost always the right one.

• extract_ticket_search_terms() — regex-extracts distinctive identifiers (filenames, kebab/camel/Pascal/snake_case), deduped and capped.
• select_repo() — probes up to 4 terms across the candidate repos (repos ≤ 10), tallies matches per repo, and either short-circuits to a uniquely-strong match or feeds the evidence into the LLM prompt ("STRONGLY prefer the repo with concrete matches").

Hardening (from adversarial review)

• Resolve each repo's default branch instead of passing "HEAD" — "HEAD" 404s the snapshot ref lookup and silently falls back to the rate-limited Code Search API (up to 40 probes/triage → 403 after ~10 → corrupted evidence). This was a HIGH bug that would've silently defeated the feature.
• Short-circuit requires ≥2 distinct identifiers agreeing on one repo; a single generic token is too weak to bypass the LLM (could re-trigger the exact mis-route).
• Log search errors; if searches fail systemically, drop the evidence rather than present "no matches" as fact.

Verification

• cargo clippy -p worker --all-targets -D warnings — clean
• cargo fmt --check — clean
• cargo test -p worker — new test: extractor catches TICKET-7881 (tracking-tighter, letter-spacing) and TICKET-7838 (ClassRow, GroupCard) identifiers; yields nothing for a prose-only ticket (clean LLM fallback)
• End-to-end: after deploy I'll re-run TICKET-7881 and confirm it now routes to web-app.

Follow-up (tracked, not in this PR)

Selection snapshot-searches each candidate repo (downloads tarballs during triage). An org-scoped Code Search (org: qualifier, one call per term) would avoid the per-triage downloads. Memory: the repos ≤ 10 fan-out holds several snapshots; fine for normal repos, bounded by the tarball cap + API fallback for outliers.