#20706: feat(hooks): add outputDir config for session-memory hook

Adds optional \`outputDir\` config so session summaries can be written outside \`memory/\` (e.g. to \`session-summaries/\`) and stay out of the vector index. <!-- greptile_comment --> <h3>Greptile Summary</h3> Adds an optional `outputDir` configuration to the session-memory hook so session summaries can be written to a custom workspace-relative directory (e.g. `session-summaries/`) instead of always going to `memory/`. This allows users to keep session summaries out of the vector memory index while maintaining security by falling back to `memory/` if the configured path would escape the workspace boundary. **Key changes:** - Added path validation logic in `handler.ts:188-211` that resolves the configured `outputDir` relative to workspace, checks it stays within workspace bounds using `realpath` and path prefix checking, and falls back to `memory/` with a warning if it would escape - Updated documentation to explain the new config option and security behavior - Added comprehensive test coverage for both the happy path (custom directory) and security fallback (path traversal attempts) <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation includes proper security validation to prevent directory traversal attacks, comprehensive test coverage for both normal operation and edge cases, clear documentation, and follows the repository's coding patterns. The path validation logic correctly uses realpath and prefix checking to ensure the output directory stays within workspace boundaries. - No files require special attention _{Last reviewed commit: a7e2898} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->