#21105: feat(matrix): multi-account support

## Summary This PR adds first-class multi-account support to the Matrix extension by introducing an `accounts` map in the Matrix config and threading `accountId` through client resolution, monitoring, and outbound/actions. It also adds an active-client registry keyed by account so outbound actions can reuse the correct running client, while keeping the shared client lifecycle safe across concurrent starts. ## Backward compatibility Legacy single-account configuration remains supported: existing top-level Matrix fields (`homeserver`, `userId`, `accessToken`, etc.) still parse and continue to work without an `accounts` section. When no explicit `accountId` is provided, the default account key is used. ## Test coverage - Unit tests added for Matrix config schema parsing (legacy + multi-account + invalid policy cases). - Unit tests added for the per-account active-client registry behavior. Note: local integration/e2ee tests that depend on the `matrix-sdk-crypto` native module cannot be exercised here because the native dependency is not installed in this environment; unit tests and build pass. ## Files changed - `extensions/matrix/src/config-schema.ts` - `extensions/matrix/src/matrix/client/shared.ts` - `extensions/matrix/src/matrix/active-client.ts` - `extensions/matrix/src/matrix/monitor/index.ts` - `extensions/matrix/src/matrix/monitor/handler.ts` New tests: - `extensions/matrix/src/config-schema.test.ts` - `extensions/matrix/src/matrix/active-client.test.ts` <!-- greptile_comment --> <h3>Greptile Summary</h3> Introduced multi-account support for Matrix by adding an `accounts` map to the config schema while preserving backward compatibility with legacy single-account configuration. The implementation threads `accountId` through client resolution, monitoring, actions, and outbound handlers. Storage paths are now account-scoped to prevent conflicts between multiple accounts on the same homeserver. An active-client registry keyed by account ID enables concurrent operation of multiple Matrix accounts while maintaining safe shared client lifecycle management. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation follows clean patterns with comprehensive test coverage for config parsing and active-client registry. Backward compatibility is properly maintained through fallback logic, and the accountId threading is consistently applied across all components - No files require special attention _{Last reviewed commit: 9a98922} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->