feat(ultraplan-local): emit main-merge-gate stats event from Phase 8

Wire the main-merge-gate lifecycle event into commands/ultraexecute-local.md
Phase 8. Three event variants emitted via lib/stats/event-emit.mjs (S8):
  - main-merge-gate     fired at the gate boundary
  - main-merge-approved fired on operator confirm
  - main-merge-declined fired on operator decline (run recorded as partial)

The gate ALWAYS pauses regardless of gates_mode — it is the one always-on
boundary that --gates does not toggle. On decline, --resume re-enters at
the gate, and the wave session branches survive on the remote thanks to
Hard Rule 19's push-before-cleanup. Recovery surface is documented inline.

Pin in tests/lib/main-merge-gate.test.mjs locks the always-on prose, the
event names, and the recovery-surface contract.

plugins/ultraplan-local/tests/lib/main-merge-gate.test.mjs

@@ -0,0 +1,42 @@
+// tests/lib/main-merge-gate.test.mjs
+// Step 12 (plan-v2) — pin that commands/ultraexecute-local.md Phase 8
+// names the main-merge-gate lifecycle event, the decline + recovery
+// surface, and the always-on gate prose.
+
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { readFileSync } from 'node:fs';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const HERE = dirname(fileURLToPath(import.meta.url));
+const ROOT = join(HERE, '..', '..');
+const CMD = readFileSync(join(ROOT, 'commands/ultraexecute-local.md'), 'utf-8');
+
+test('Phase 8 names the main-merge-gate lifecycle event', () => {
+  assert.ok(
+    CMD.includes('main-merge-gate'),
+    'commands/ultraexecute-local.md should emit `main-merge-gate` from Phase 8',
+  );
+});
+
+test('Phase 8 documents both approved + declined event branches', () => {
+  assert.ok(CMD.includes('main-merge-approved'), 'should emit main-merge-approved on confirm');
+  assert.ok(CMD.includes('main-merge-declined'), 'should emit main-merge-declined on decline');
+});
+
+test('Phase 8 documents the --resume recovery surface for the main-merge gate', () => {
+  assert.ok(
+    CMD.includes('--resume re-enters'),
+    'Phase 8 should document that `--resume re-enters at the gate` after a decline',
+  );
+});
+
+test('Phase 8 main-merge gate is always-on (regardless of gates_mode)', () => {
+  // Main-merge gate is the one boundary that pauses on every run; the prose
+  // must say so explicitly so the contract survives copy-edit drift.
+  assert.ok(
+    /always[\s\S]{0,200}gates_mode|gates_mode[\s\S]{0,200}always|always pauses on every run/.test(CMD),
+    'Phase 8 should state main-merge gate is always-on, regardless of gates_mode',
+  );
+});