docs(examples): add runnable index sync example (updates, deletes, folder sync) (#199) - #201

Summary

Adds examples/index_sync.py, a runnable, standalone script demonstrating the three core patterns for keeping a Whoosh index synchronized with changing source data:

1. Upsert (update_document): Shows that indexing on a unique=True ID field replaces an existing document rather than duplicating it, while adding new documents seamlessly.
2. Delete (delete_by_term): Removes documents by unique key and confirms they are completely excluded from search results.
3. Folder Sync (sync_folder): A 3-pass reconciliation demo comparing stored file modification times against files on disk:
   • Pass 1 (Initial sync): indexes new files.
   • Pass 2 (Incremental sync): detects modified files, adds new files, and deletes removed files in a single atomic commit.
   • Pass 3 (Idempotent check): confirms zero work is done when files haven't changed.

Also:

• Added an ## Index management section to examples/README.md housing index_sync.py and resource_management.py.
• Added "index_sync.py" to the _RUNNABLE allowlist in tests/test_examples_runnable.py so CI automatically smoke-tests it end-to-end.
• Updated CHANGELOG.md under ## [Unreleased].

Related issues

Fixes #199

Checklist

• Tests pass locally (pytest tests/)
• Added/updated tests for the change where it makes sense
• Updated docs / CHANGELOG if user-facing
• I have read the CONTRIBUTING guide

Authored with AI assistance via Antigravity (DeepMind agentic coding system) with complete human validation, testing, and DCO sign-off.