chore(tracker): sync Step 1.5 → ✅ and log PRs #61, #62

[officialCodeWork]

Summary

Tracker housekeeping after PR #62 (Step 1.5 — Structure-aware chunker) merged to `main`.

• Flip Step 1.5 row from ⏳ to ✅ with full deliverable summary (rag-chunker package, Chunker SPI, parent-link semantics via heading stack, token-budgeted body chunks, within-section overlap, sentence-boundary fallback, OCR adapter, ragctl chunk command).
• Phase 1 progress 10/16 → 11/16 done (5 remaining); Total 23/84 → 24/84.
• Advance Next-action line to Step 1.6 (metadata enricher), noting it operates on chunk output from Step 1.5 before Step 1.7 PII and Step 1.8 embedding.
• Add PRs chore(tracker): sync Step 1.4 → ✅ and log PRs #58, #59, #60 #61 (Step 1.4 tracker sync) and feat(core,chunker): structure-aware chunker (Step 1.5) #62 (Step 1.5) to the PR & Branch History table.

No source or test changes — pure tracker sync, matches the established pattern of PRs #49, #53, #58, #61.

Documentation

• TRACKER.md — updated as described above.

Test plan

• No code touched; lint/test status is identical to `main`.

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