feat(land): enqueue-and-return default + first-time trunk.io onboarding

On a merge queue, /land now enqueues the PR and returns by default, so you can
/land a stack of ready PRs and walk away while the queue lands them. --watch
opts into the blocking poll (and is what /land-and-deploy uses, since it deploys
the result). No-queue repos still merge synchronously.

Also: /land explains in plain English what a merge queue is and what it'll do
before submitting (teacher mode on first encounter), and when no queue is
configured it offers to set trunk.io up and hand-holds the whole flow. The
onboarding lives in one shared {{MERGE_QUEUE_SETUP}} resolver included by both
/land and /setup-deploy (DRY).

Atomic .tmpl + gen:skill-docs regen. The postfail-ordering test is updated in
the same commit because 4.3 was renamed (Wait -> enqueue/watch); it now matches
the section number so the rename stays bisect-clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/land-and-deploy-postfail.test.ts

@@ -39,13 +39,15 @@ describe("PR #1620 post-failure PR-state check in /land template", () => {
     expect(readTmpl()).toMatch(/### 4\.2a: Post-failure PR-state check/);
   });
 
-  test("post-failure check comes before the wait step", () => {
+  test("post-failure check comes before the landing step (4.3)", () => {
     const body = readTmpl();
     const postfail = body.indexOf("### 4.2a: Post-failure PR-state check");
-    const wait = body.indexOf("### 4.3: Wait for it to land");
+    // 4.3 is the landing step (enqueue-and-return by default, or --watch). Match
+    // the section number, not its title, so D4's rename doesn't break the order check.
+    const landing = body.indexOf("### 4.3:");
     expect(postfail).toBeGreaterThan(-1);
-    expect(wait).toBeGreaterThan(-1);
-    expect(postfail).toBeLessThan(wait);
+    expect(landing).toBeGreaterThan(-1);
+    expect(postfail).toBeLessThan(landing);
   });
 
   test("Universal invariant + upstream gh bug references", () => {