chore(rag): remove the dead populateRepoIndexStub / INDEX_JOB_FOLLOWUP artifacts

[JSONbored]

Summary

populateRepoIndexStub + the INDEX_JOB_FOLLOWUP marker in rag-wire.ts were a documented stub + ticket left from when RAG retrieval shipped ahead of index population. Population has since been fully implemented in rag-index.ts (fetch tree → chunk → embed → upsert, scheduled from the six-hourly cron fan-out + merged-PR webhooks).

So the stub is dead code that only misled readers into thinking RAG was inert (it tripped up a recent audit). Remove the stub + its test and correct the now-stale comments to point at the real implementation.

Validation

• npm run typecheck; npm run test:coverage — full suite green; deletion-only on src/** (no new coverage obligation; the removed code's only test is removed with it). Symbols (upsertChunks/splitRepo/createReviewAdapters) remain used elsewhere, so no unused imports.

Roadmap item #12 (hygiene).

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.44%. Comparing base (6543e3c) to head (e0eb60e).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1484      +/-   ##
==========================================
- Coverage   95.44%   95.44%   -0.01%     
==========================================
  Files         194      194              
  Lines       21070    21066       -4     
  Branches     7624     7624              
==========================================
- Hits        20110    20106       -4     
  Misses        383      383              
  Partials      577      577              

Files with missing lines	Coverage Δ
src/review/rag-index.ts	89.65% <ø> (ø)
src/review/rag-wire.ts	93.10% <ø> (-0.84%)	⬇️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.