feat(fleet): onboarding-injection — comms cheat-sheet + peer roster per agent (#620)

Root cause of a spawned agent's failed first send: fleet agents are born not
knowing how to reach their peers. Inject a comms cheat-sheet + peer roster into
each fleet agent's system prompt via composeContract (the runtime-agnostic path
every `mosaic yolo <runtime>` agent hits), so it can reach the orchestrator and
peers from its first turn.

- src/fleet/comms-onboarding.ts (standalone, no fleet.ts coupling):
  parseRosterAgents (name/class/host/ssh/socket), renderPeerReach (data-driven
  agent-send command), buildFleetCommsBlock (own [host:session] identity +
  agent-send path + peer table + FLIP-to-reply + `agent send --verify`=ACCEPTED),
  readFleetCommsBlock (reads roster.yaml; '' when not a member).
- launch.ts composeContract appends it only when MOSAIC_AGENT_NAME is set + the
  agent is in the roster (no-op for non-fleet launches).
- roster.schema.json: optional per-agent host/ssh/socket (cross-host + socket
  addressing; manual cross-host listing is a pre-federation stopgap).

Address rendering is fully data-driven per Mos's acceptance criteria:
- cross-host: local → `-s <session>` (no -H); remote → `-H <ssh> -s <session>`.
- socket: named → `-L <socket>`; unset → default socket (no -L) — matches the
  LIVE socket, never blindly inherits the roster's tmux.socket_name.

Verified: 14 onboarding + 9 composeContract tests (parse, render local/remote/
fallback/equal-host/socket-none/named/combined, build, situational read +
composeContract injection w/ correct cross-host+socket addrs, no-op when
MOSAIC_AGENT_NAME unset). tsc/eslint/prettier/sanitize clean.

Refs #620

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

@@ -81,12 +81,27 @@
           "class": {
             "type": "string"
           },
+          "host": {
+            "description": "Host the agent runs on (hostname or IP). Absent = the fleet host. Used by onboarding-injection to render cross-host comms addresses. Manual cross-host listing is a pre-federation stopgap; federation (W1) auto-discovers later.",
+            "type": "string"
+          },
+          "ssh": {
+            "description": "SSH target (user@host) for a cross-host peer, so onboarding renders the `agent-send.sh -H <user@host>` form. Optional; only needed for agents on a different host than the fleet.",
+            "type": "string"
+          },
+          "socket": {
+            "description": "tmux socket the agent's session runs on. Onboarding renders `-L <socket>` when set; absent = the default socket (no `-L`). Must match the LIVE socket, not blindly inherit the roster's tmux.socket_name.",
+            "type": "string"
+          },
           "working_directory": {
             "type": "string"
           },
           "workingDirectory": {
             "type": "string"
           },
+          "workingDirectory": {
+            "type": "string"
+          },
           "model_hint": {
             "type": "string"
           },