✨ Add create:finish merge front door

The merge half of the branching model was still prose, so it drifted per
session. create:finish mirrors create:branch: it asserts the merge-side
preconditions, fast-forwards a stale main (divergence is refused), merges
--no-ff, runs npm run verify, and deletes the branch only when green. The
push stays with create:release so the merge is reviewable first.

AGENTS.md

@@ -51,7 +51,7 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
     **Known problems:** <open issues, caveats, follow-ups>
     ```
 
-    Once the user has no further objections, merge back: `git checkout main && git merge --no-ff <branch>`. The branching model is documented in [CONTRIBUTING.md § Branching model](./CONTRIBUTING.md#branching-model).
+    Once the user has no further objections, merge back: `npm run create:finish` (on the branch — it merges `--no-ff`, runs `npm run verify`, and deletes the branch). The branching model is documented in [CONTRIBUTING.md § Branching model](./CONTRIBUTING.md#branching-model).
 
 - **Leaf task** (no indented children): implement on the current branch and commit.