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

packages/mosaic/framework/fleet/roster.schema.json

@@ -113,6 +113,35 @@
           }
         }
       }
+    },
+    "connector": {
+      "description": "Orchestrator chat connector (F4). Optional — absent means tmux (back-compat). Secrets (access/bot tokens) come from the environment, never this file.",
+      "type": "object",
+      "additionalProperties": false,
+      "required": ["kind"],
+      "properties": {
+        "kind": {
+          "enum": ["tmux", "discord", "matrix"]
+        },
+        "matrix": {
+          "type": "object",
+          "additionalProperties": false,
+          "required": ["homeserver_url", "user_id", "room_id"],
+          "properties": {
+            "homeserver_url": { "type": "string" },
+            "user_id": { "type": "string" },
+            "room_id": { "type": "string" }
+          }
+        },
+        "discord": {
+          "type": "object",
+          "additionalProperties": false,
+          "required": ["channel_id"],
+          "properties": {
+            "channel_id": { "type": "string" }
+          }
+        }
+      }
     }
   }
 }