feat(fleet): F4 Phase 1 — orchestrator chat connector abstraction + Matrix design (#616)

From the north-star (#613) orchestrator-chat-connector decision: make the
orchestrator's chat channel a pluggable, user-chosen connector (tmux | discord
| matrix peers) and add Matrix (local homeserver) — starting with the small
uniform abstraction so connectors drop in without touching fleet core.

Phase 1 (this PR — design + scaffold):
- src/fleet/connectors/types.ts: OrchestratorConnector interface (send /
  subscribe / health per the Lead's spec) + message/config types; thread-aware
  via optional threadId so Matrix threads + the future first-party Mosaic
  Discord plugin fit without an interface change. DEFAULT_CONNECTOR_KIND=tmux.
- src/fleet/connectors/registry.ts: extensible factory registry;
  resolveConnectorKind defaults tmux (back-compat); createConnector throws
  ConnectorNotImplementedError until Phase 2 registers factories.
- roster.schema.json: optional connector block (tmux|discord|matrix; matrix
  homeserver/user/room). Secrets via env (gateway pattern), never the roster.
- docs/fleet/f4-matrix-connector.md: interface, config, Matrix client-server API
  mapping, Conduit-default local homeserver, phasing, back-compat.

Self-contained connectors/ module — NO fleet.ts changes, so independent of the
in-flight stacked fleet-config PR (#615).

Verified: 7 connector tests green; tsc/eslint/prettier/sanitize clean; schema
valid JSON.

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

@@ -0,0 +1,19 @@
+# F4 — Orchestrator chat connector + Matrix (#616)
+
+- **Issue:** #616 · **Branch:** `feat/f4-matrix-connector` (off main; independent of #615) · **Doctrine:** north-star #613.
+
+## Phase 1 (this PR) — abstraction + scaffold
+
+- `src/fleet/connectors/types.ts`: `OrchestratorConnector` (send/subscribe/health) + message/config types; thread-aware via optional `threadId`; `DEFAULT_CONNECTOR_KIND=tmux`.
+- `src/fleet/connectors/registry.ts`: extensible factory registry; `resolveConnectorKind` (defaults tmux, back-compat); `createConnector` throws `ConnectorNotImplementedError` until Phase 2 registers factories.
+- `roster.schema.json`: optional `connector` block (tmux|discord|matrix; matrix homeserver/user/room; secrets via env, never roster).
+- Design doc `docs/fleet/f4-matrix-connector.md`: interface, config, Matrix CS-API mapping, Conduit-default infra, phasing.
+- **No fleet.ts changes** → self-contained, zero conflict with stacked #615.
+
+## Verification
+
+- 7 connector tests green; tsc/eslint/prettier/sanitize clean; schema valid JSON.
+
+## 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.