fix: preserve complete README_AI sections when truncating

[dreamlx]

Closes #178

Summary

• retain the 10KB navigation budget, including the truncation notice
• truncate at complete Markdown heading boundaries (## through ######), so detailed ### file symbol lists are never cut mid-item
• replace the misleading per-module README hint with a source-reading next step
• cover detailed-section regression and strict byte-budget behavior

Verification

• pytest tests/writers/test_utils.py tests/test_smart_writer.py -q — 37 passed
• ruff check src/ tests/ — passed
• full suite with current source in a clean test worktree — 1788 passed, 13 skipped

Note: running the full suite from this developer checkout is contaminated by its ignored .codeindex.yaml (it enables extra languages and invalidates four default-language tests); the clean-worktree run avoids changing that user-local config.