fix(fleet): additive hook-event merge and gated-composition acceptance reads

Integration adjudication (fred, W-F1): the general arrays-replace merge rule
conflicts with the gap-7 base/overlay split — base and lease overlay share
the PreToolUse and Stop events, so replace semantics would silently drop the
base QA hooks from every gated seat. Ruling: hook event arrays directly
under the top-level hooks key concatenate (base first); all other arrays
keep replace semantics; null tombstones still delete an event.

- mutator-gate acceptance now asserts lease wiring against the COMPOSED
  gated settings (base + lease-overlay via the launcher's own merge),
  matching the post-split contract.
- fleet subcommand canary gains the intended new 'agent' surface from T3.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Dtdjx4Gxude9fwyLezCrhh

packages/mosaic/src/commands/fleet-launch-command.spec.ts

@@ -94,6 +94,36 @@ describe('three-layer settings merge', () => {
     ).toEqual({ nested: { keep: true } });
   });
 
+  it('concatenates hook event arrays so an overlay adds gating without erasing base hooks', () => {
+    const qaStop = { hooks: [{ type: 'command', command: 'qa-stop.sh' }] };
+    const leaseStop = { hooks: [{ type: 'command', command: 'receipt-observer.py' }] };
+    const qaPre = { matcher: 'Write', hooks: [{ type: 'command', command: 'qa-pre.sh' }] };
+    expect(
+      deepMergeSettings(
+        { hooks: { Stop: [qaStop], PreToolUse: [qaPre] } },
+        { hooks: { Stop: [leaseStop] } },
+      ),
+    ).toEqual({ hooks: { Stop: [qaStop, leaseStop], PreToolUse: [qaPre] } });
+  });
+
+  it('still deletes a whole hook event via the null tombstone', () => {
+    expect(
+      deepMergeSettings(
+        { hooks: { Stop: [{ hooks: [{ type: 'command', command: 'qa-stop.sh' }] }] } },
+        { hooks: { Stop: null } },
+      ),
+    ).toEqual({ hooks: {} });
+  });
+
+  it('keeps replace semantics for arrays outside the top-level hooks object', () => {
+    expect(
+      deepMergeSettings(
+        { plugins: ['base'], nested: { hooks: { Stop: ['base'] } } },
+        { plugins: ['user'], nested: { hooks: { Stop: ['user'] } } },
+      ),
+    ).toEqual({ plugins: ['user'], nested: { hooks: { Stop: ['user'] } } });
+  });
+
   it('deep-merges all three layers in precedence order', () => {
     expect(
       deepMergeSettings(