feat(fleet): comms-block emitter + FLEET-LAUNCH runbook (#633) (#638)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

packages/mosaic/src/fleet/comms-onboarding.ts

@@ -179,5 +179,48 @@ export function readFleetCommsBlock(
   });
 }
 
+/** Result of resolving a comms-block emit request — see `mosaic fleet comms-block`. */
+export interface CommsBlockResult {
+  /** True when a cheat-sheet was produced; false maps to stderr + non-zero exit. */
+  ok: boolean;
+  /** The Fleet-Comms cheat-sheet (empty unless ok). */
+  output: string;
+  /** Operator-facing reason when !ok. */
+  error?: string;
+}
+
+/**
+ * Resolve the Fleet-Comms cheat-sheet for an explicit <role>, backing the
+ * `mosaic fleet comms-block <role>` command. Unlike readFleetCommsBlock — which
+ * returns '' on any miss so composeContract can no-op silently during a launch —
+ * this NEVER silently emits empty: an unknown role or missing roster yields
+ * ok:false + an operator-facing reason, so the CLI surfaces it (stderr + exit 1)
+ * rather than printing nothing. That makes it safe to preview any peer's view,
+ * e.g. `mosaic fleet comms-block coder0-0`.
+ */
+export function resolveCommsBlock(
+  mosaicHome: string,
+  role: string | undefined,
+  fleetHost?: string,
+): CommsBlockResult {
+  if (!role) {
+    return { ok: false, output: '', error: 'comms-block requires a <role> argument' };
+  }
+  const block = fleetHost
+    ? readFleetCommsBlock(mosaicHome, role, fleetHost)
+    : readFleetCommsBlock(mosaicHome, role);
+  if (!block) {
+    const rosterPath = join(mosaicHome, 'fleet', 'roster.yaml');
+    return {
+      ok: false,
+      output: '',
+      error: existsSync(rosterPath)
+        ? `role "${role}" is not a member of the fleet roster at ${rosterPath}`
+        : `no fleet roster at ${rosterPath}`,
+    };
+  }
+  return { ok: true, output: block };
+}
+
 /** Default mosaic home (mirrors launch.ts), for callers that don't pass one. */
 export const DEFAULT_MOSAIC_HOME_FOR_COMMS = join(homedir(), '.config', 'mosaic');