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/scratchpads/f4-matrix-connector.md

@@ -17,3 +17,14 @@
 ## Phase 2+ (follow-ups, in the doc)
 
 Matrix CS-API client (fetch send/sync/health) + factory; init/configure connector-selection UX + roster-parse wiring; systemd launch wiring; Conduit deploy guide; first-party Mosaic Discord (threads) as a connector.
+
+## Phase 2a (feat/f4-matrix-client, stacked on #617) — Matrix CS-API client
+
+- `src/fleet/connectors/matrix.ts`: `MatrixConnector implements OrchestratorConnector` over the Matrix
+  client-server API (injectable fetch, no SDK). `send` → PUT m.room.message (thread-aware); `subscribe`
+  → /sync long-poll loop using the pure `parseSyncResponse`; `health` → /versions + /whoami.
+  `registerMatrixConnector(env)` registers the factory (token from MATRIX_ACCESS_TOKEN, never roster).
+- Pure helpers `buildMessageBody` + `parseSyncResponse` make send/receive unit-testable.
+- 13 Matrix tests + 7 registry = 20 connector tests green; tsc/eslint/prettier clean.
+- Remaining Phase 2: init/configure connector-selection UX + roster-parse wiring (touches fleet.ts —
+  after #615); systemd launch wiring; Conduit deploy guide.