Implements the Matrix connector behind the F4 abstraction (stacked on #617). Speaks the Matrix client-server API directly over HTTPS (injectable fetch, no SDK) so it is homeserver-agnostic (Conduit default, Synapse alt). - MatrixConnector implements OrchestratorConnector: - send → PUT /_matrix/client/v3/rooms/{room}/send/m.room.message/{txn} (thread-aware via m.thread relation), returns event_id. - subscribe → /sync long-poll loop driven by the pure parseSyncResponse (skips the orchestrator's own echoes; carries threadId). - health → /_matrix/client/versions (reachable) + /account/whoami (authed). - Pure helpers buildMessageBody + parseSyncResponse → send/receive unit-testable. - registerMatrixConnector(env) registers the factory; the access token comes from MATRIX_ACCESS_TOKEN, never the roster. Verified: 13 Matrix + 7 registry = 20 connector tests green; tsc/eslint/prettier clean. Still a self-contained connectors/ module (no fleet.ts changes). Stacked on #617 (F4 Phase 1). Merge order: #617 -> this; rebases onto main after. Refs #616, #613 Co-Authored-By: Claude Opus 4.8 <[email protected]> Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83