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

docs/TASKS.md

@@ -70,3 +70,7 @@ Active workstream is **W1 — Federation v1**. Workers should:
 ## F4 — Orchestrator chat connector + Matrix (#616) — feat/f4-matrix-connector
 
 - Status: Phase 1 MERGED (#617: connector interface send/subscribe/health + registry + roster schema + design). Phase 2a (#618): Matrix CS-API client + factory. 20 connector tests green; no fleet.ts changes. Remaining Phase 2: init/configure connector-selection UX + roster wiring, systemd launch wiring, Conduit deploy guide. Detail: scratchpads/f4-matrix-connector.md.
+
+## Fleet onboarding-injection — comms cheat-sheet + peer roster (#620) — feat/fleet-comms-onboarding
+
+- Status: implemented + tested. Injects # Fleet Comms (peer roster + cross-host agent-send commands + FLIP-reply + --verify) into each spawned fleet agent via composeContract; optional per-agent host/ssh/socket roster fields (socket: named → -L, unset → default socket no -L). 10 + 2 tests green. Detail: scratchpads/fleet-comms-onboarding.md.