Add patch command with incremental single-document reindex

[G9Pedro]

Summary

• add a new clawvault patch <idOrPath> command for in-place document updates
• support three patch modes:
  • --append <text>
  • --replace <text> --with <text>
  • --section <heading> --content <text>
• add strict CLI validation so exactly one patch mode is selected and --with is only used with --replace
• implement ClawVault.patch() with section-aware markdown patching and path safety checks
• update indexing behavior to refresh only the patched document in the local index (no full reindex() call)
• extend tests across core vault behavior and CLI registration/forwarding

Testing

• npm run build
• npm test
• manual CLI walkthrough demo of all patch modes (append, replace/with, section/content) with final get verification