fix(observability): surface RAG index-population failures to Sentry

[JSONbored]

Summary

Extends the review-pipeline Sentry coverage (after #1619 covered RAG retrieval / enrichment / inline failures) to the RAG index-population path. fetchRepoTree and listStoredChunkPaths (rag-index.ts) logged their catch failures via console.log with no level, so the central Sentry forwarder (which forwards only level:"error"/"fatal") never saw them — a broken index population (GitHub tree fetch down, D1 read error) silently degraded RAG quality with no Sentry signal.

Promote both to level:"error" event:"rag_index_tree_error" / "rag_list_paths_error" (keeping the ev tag for log continuity), mirroring the #1619 rag.ts pattern. Both remain fail-safe (population still degrades gracefully) — only visibility changes.

Extends #1618. (The two outer-catch population sites — rag_index_repo_error / rag_reindex_paths_error — are defensive/unreachable in the current tests and are a separate follow-up with their own coverage.)

Scope

• Conventional Commit title; focused (one file + its test).
• No site//CNAME/Pages; follows CONTRIBUTING.md.
• Small self-evident extension of Review pipeline silently degrades on RAG / enrichment / inline-comment failures (not surfaced to Sentry) #1618/fix(observability): surface RAG/enrichment/inline review failures to Sentry (#1618) #1619 (no separate issue needed).

Validation

• git diff --check · actionlint · typecheck
• test:coverage — both promoted catches are exercised by the existing rag-index tests (tree-throws; stored-paths read error), now asserting the level:"error" log; no new branches.
• test:workers · build:mcp · test:mcp-pack · ui:* · npm audit --audit-level=moderate

If any required check was skipped, explain why:

• No migration/OpenAPI/cf-typegen: logging-level change only.

Safety

• No secrets/wallets/trust-scores exposed — logs carry repo/project + a bounded error message.
• No public GitHub text change · auth/CORS N/A.

Notes

• This is self-host engine code (RAG index population runs on the container); it ships on the next self-host rsync+rebuild (logging-only, non-urgent — can batch with other self-host changes).

[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 95.51%. Comparing base (031cb7b) to head (9ef54e4).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1625   +/-   ##
=======================================
  Coverage   95.51%   95.51%           
=======================================
  Files         204      204           
  Lines       22051    22051           
  Branches     7966     7966           
=======================================
  Hits        21062    21062           
  Misses        413      413           
  Partials      576      576           

Files with missing lines	Coverage Δ
src/review/rag-index.ts	89.65% <100.00%> (ø)

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