feat(fleet): F4 Phase 2a — Matrix CS-API connector client + factory (#616)

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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

docs/TASKS.md

@@ -61,4 +61,4 @@ Active workstream is **W1 — Federation v1**. Workers should:
 
 ## F4 — Orchestrator chat connector + Matrix (#616) — feat/f4-matrix-connector
 
-- Status: Phase 1 done (abstraction + scaffold). Connector interface (send/subscribe/health) + registry + roster connector schema + design doc; tmux default/back-compat; matrix/discord factories are Phase 2. 7 tests green; no fleet.ts changes (independent of #615). Detail: scratchpads/f4-matrix-connector.md.
+- Status: Phase 1 done (abstraction + scaffold). Connector interface (send/subscribe/health) + registry + roster connector schema + design doc; tmux default/back-compat; matrix factory + CS-API client landed (Phase 2a, #617-stacked); 20 connector tests green; no fleet.ts changes (independent of #615). Detail: scratchpads/f4-matrix-connector.md.