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>

setup-deploy/SKILL.md.tmpl

@@ -220,6 +220,13 @@ section if it exists, or append it. Keep it separate from Deploy Configuration s
 `/land` reads the `Merge queue:` line to pick its submit path. If you skip this section,
 `/land` falls back to live detection and asks once.
 
+**If the user chose `trunk` and the queue isn't set up yet** (no `Trunk Merge Queue (<base>)`
+check on recent PRs — check with `gh pr checks` or `gh api`), walk them through the
+one-time onboarding below before writing `Merge queue: trunk`. If they chose `none` or
+`github`, skip the onboarding.
+
+{{MERGE_QUEUE_SETUP}}
+
 ### Step 5: Verify
 
 After writing, verify the configuration works: