✨ 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.

package.json

@@ -49,6 +49,7 @@
         "fix:oxfmt": "oxfmt ${LEFTHOOK_FILES:-.}",
         "fix:oxlint": "oxlint --fix src scripts",
         "create:branch": "./scripts/branch.sh",
+        "create:finish": "./scripts/finish.sh",
         "create:release": "./scripts/release.sh",
         "maintain": "npm run maintain:knip; npm run maintain:outdated",
         "maintain:knip": "knip --include dependencies,exports,files",