feat(index): add file watcher for incremental code index updates

[bug-ops]

Summary

• Add IndexWatcher in zeph-index using notify_debouncer_mini with 1s debounce, following SkillWatcher pattern
• On file change, sends path via mpsc channel to consumer task that calls CodeIndexer::reindex_file()
• Filters by indexable file extensions, deduplicates events via HashSet
• New watch: bool config field in [index] section (default true)
• Watcher lives in outer main scope for full program lifetime

Test plan

• Existing test suite passes (1278 passed, 9 skipped)
• start_with_valid_directory — happy path test
• start_with_nonexistent_directory_fails — error path test
• clippy clean, fmt pass

Closes #233

[codecov-commenter]

Codecov Report

❌ Patch coverage is 41.23711% with 57 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-index/src/watcher.rs	47.36%	40 Missing ⚠️
src/main.rs	0.00%	17 Missing ⚠️

❌ Your patch status has failed because the patch coverage (41.23%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
- Coverage   80.96%   80.87%   -0.10%     
==========================================
  Files          78       79       +1     
  Lines       23137    23232      +95     
==========================================
+ Hits        18734    18789      +55     
- Misses       4403     4443      +40     

Files with missing lines	Coverage Δ
crates/zeph-core/src/config.rs	98.92% <100.00%> (+<0.01%)	⬆️
src/main.rs	70.01% <0.00%> (-0.85%)	⬇️
crates/zeph-index/src/watcher.rs	47.36% <47.36%> (ø)

... and 2 files with indirect coverage changes

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