chore(tracker): sync Step 2.1 → ✅ and log PRs #73, #74

[officialCodeWork]

Summary

• Mark Phase 2 Step 2.1 (Knowledge store read layer) as ✅ and link PR #74.
• Add PR History entries for #73 (prior tracker sync) and #74.
• Stamp Last updated with the sync note. Phase 2 done count was already 1 and remains correct.

Test plan

• TRACKER.md row 2.1 shows ✅ with PR feat(core,backends): knowledge store read layer + FilterExpr push-down (Step 2.1) #74 link
• PR history table contains rows for chore(tracker): mark Phase 1 complete (Step 1.10 → ✅; log PRs #71, #72) #73 and feat(core,backends): knowledge store read layer + FilterExpr push-down (Step 2.1) #74
• Milestone count for Phase 2 (1 of 11) still matches the marked-✅ rows
• No changes outside TRACKER.md

Documentation

• TRACKER.md — the doc being updated.

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