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.ts

@@ -253,9 +253,12 @@ function cloneValue(value: unknown): unknown {
   return value;
 }
 
+type MergeContext = 'root' | 'hooks' | 'nested';
+
 function mergeObject(
   lower: Record<string, unknown>,
   higher: Record<string, unknown>,
+  context: MergeContext = 'nested',
 ): Record<string, unknown> {
   const result = cloneValue(lower) as Record<string, unknown>;
   for (const [key, highValue] of Object.entries(higher)) {
@@ -264,20 +267,36 @@ function mergeObject(
       continue;
     }
     const lowValue = result[key];
+    if (context === 'hooks' && Array.isArray(lowValue) && Array.isArray(highValue)) {
+      result[key] = [...(lowValue as unknown[]), ...(cloneValue(highValue) as unknown[])];
+      continue;
+    }
     result[key] =
       isPlainObject(lowValue) && isPlainObject(highValue)
-        ? mergeObject(lowValue, highValue)
+        ? mergeObject(
+            lowValue,
+            highValue,
+            context === 'root' && key === 'hooks' ? 'hooks' : 'nested',
+          )
         : cloneValue(highValue);
   }
   return result;
 }
 
-/** Deep object merge. Scalars and arrays replace; null in a higher layer deletes. */
+/**
+ * Deep object merge. Scalars and arrays replace; null in a higher layer
+ * deletes. Exception: hook event arrays directly under the top-level `hooks`
+ * key concatenate (base entries first), so an overlay ADDS gating without
+ * erasing the base QA hooks that share an event — replacing them would make
+ * the gap-7 base/overlay split unimplementable without duplicating base
+ * hooks inside the lease overlay. Removing an event entirely still works via
+ * the null tombstone.
+ */
 export function deepMergeSettings(
   ...layers: ReadonlyArray<Record<string, unknown> | undefined>
 ): Record<string, unknown> {
   return layers.reduce<Record<string, unknown>>(
-    (merged, layer) => (layer === undefined ? merged : mergeObject(merged, layer)),
+    (merged, layer) => (layer === undefined ? merged : mergeObject(merged, layer, 'root')),
     {},
   );
 }