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>

scripts/resolvers/index.ts

@@ -36,8 +36,10 @@ import { generateMakePdfSetup } from './make-pdf';
 import { generateTasksSectionEmit, generateTasksSectionAggregate } from './tasks-section';
 import { SECTION, SECTION_INDEX } from './sections';
 import { generateRedactTaxonomyTable, generateRedactInvocationBlock } from './redact-doc';
+import { generateMergeQueueSetup } from './merge-queue-setup';
 
 export const RESOLVERS: Record<string, ResolverValue> = {
+  MERGE_QUEUE_SETUP: generateMergeQueueSetup,
   SLUG_EVAL: generateSlugEval,
   SLUG_SETUP: generateSlugSetup,
   REDACT_TAXONOMY_TABLE: generateRedactTaxonomyTable,