chore(tracker): sync Step 2.3 → log PR #82 + history rows #77, #82

[officialCodeWork]

Summary

• Fills in the Step 2.3 PR column with [feat(backends): BM25 keyword retrieval — Elasticsearch + tantivy (Step 2.3) #82].
• Refreshes the Last updated line to "tracker sync — PR feat(backends): BM25 keyword retrieval — Elasticsearch + tantivy (Step 2.3) #82".
• Appends history rows for chore(tracker): sync Step 2.2 → log PR #76 + history rows #75, #76 #77 (the previous tracker-sync PR, post-Step 2.2) and feat(backends): BM25 keyword retrieval — Elasticsearch + tantivy (Step 2.3) #82 (the Step 2.3 implementation).

Test plan

• Tracker-only change; no code touched, no tests to run.

Documentation

• TRACKER.md — see Summary above.

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