#308 docs(whats-new): add release notes for v2.2.0

Release notes for #303 to #307 and #309: keyword-only indexes, vera index --dry-run, vera -C and the MCP repo argument (from @ple1n's use case in #302), grep -C, vera watch from subdirectories, the emptied-directory reindex fix, the BM25 writer lock fix, the broken-pipe fix, and the tokio advisory update. The heading assumes the next tag is v2.2.0 (new flags, no breaking changes). Merge after #309.

Summary by CodeRabbit

• Documentation
  • Added v2.2.0 release notes covering keyword-only and dry-run indexing, commands that target another directory or index, and shorthand for grep context.
  • The notes also describe subdirectory watch behavior, warnings for stale indexes, removal of obsolete indexes, and improved handling of locks during incremental updates.
  • They cover quiet handling when output pipes close early on Linux and macOS.