mosaic comms: socket resolution is tool-owned (B2)

--socket forwards -L verbatim when given; when OMITTED the CLI sends
no -L at all and agent-send.sh's own resolution governs (explicit -L >
MOSAIC_TMUX_SOCKET > unique hit > ambiguity refusal, B1/PR #1466).
Measured design note: comms.ts was already correct by construction
after B1 — this change pins it with a spec arm (no -L forwarded when
--socket omitted) and makes the contract explicit in the option help,
so a future default-guess here cannot silently reintroduce the
stale-twin defect. Specs 7/7.

packages/mosaic/src/commands/comms.spec.ts

@@ -15,6 +15,17 @@ import { afterEach, describe, expect, it } from 'vitest';
 import { fleetCommsSendArgs, registerCommsCommand, tmuxSendArgs } from './comms.js';
 
 describe('arg translation', () => {
+  // B2 (2026-08-29): when --socket is omitted, the CLI forwards NO -L and
+  // agent-send.sh's own resolution governs (explicit -L > MOSAIC_TMUX_SOCKET
+  // > unique hit > ambiguity refusal). Forwarding a guessed default here
+  // would defeat that resolution and reintroduce the stale-twin defect.
+  it('omits -L entirely when --socket is not given (tool-owned resolution)', () => {
+    const args = tmuxSendArgs('orch-01', 'hello', {});
+    expect(args).toEqual(['-s', 'orch-01', '-m', 'hello']);
+    expect(args).not.toContain('-L');
+    expect(args.join(' ')).not.toContain('-L');
+  });
+
   it('tmux path: -s/-C/-L/-f/-m per agent-send.sh getopts', () => {
     expect(tmuxSendArgs('orch-01', 'hello', {})).toEqual(['-s', 'orch-01', '-m', 'hello']);
     expect(

packages/mosaic/src/commands/comms.ts

@@ -62,7 +62,10 @@ export function registerCommsCommand(program: Command): void {
     .description('send <target> [message...] — same-host tmux unless --site is given')
     .option('--class <class>', 'terminal-log | actionable | human | reaction | digest')
     .option('--file <path>', 'message body from file (same-host path only)')
-    .option('--socket <name>', 'tmux socket for the same-host send (e.g. mosaic-fleet)')
+    .option(
+      '--socket <name>',
+      'tmux socket for the same-host send (e.g. mosaic-fleet). OMIT it to let agent-send.sh resolve the socket (explicit -L > MOSAIC_TMUX_SOCKET > unique hit; ambiguity refuses with its exit 4).',
+    )
     .option('--site <site>', 'route via fleet-comms to <site>/<target>')
     .option('--comms-repo <path>', 'fleet-comms checkout', defaultCommsRepo())
     .argument('<target>', 'destination seat (session name)')