✨ 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.

scripts/finish.sh

@@ -0,0 +1,136 @@
+#!/bin/sh
+
+set -eu
+
+# Feature-finish front door. Run as `npm run create:finish`.
+#
+# Why this exists: `create:branch` opens a unit of work, but the close half
+# (`git checkout main && git merge --no-ff <branch>`) stayed prose in the
+# branching model, so it drifted per contributor and per session. This is the
+# mirror image of `create:branch`: it asserts the same preconditions (clean
+# tree, no in-progress operation, `main` matching its upstream), merges the
+# current `feature/`/`fix/`/`chore/` branch into `main`, proves the result with
+# `npm run verify`, and only then deletes the branch.
+#
+# `create:branch`'s comment argued against wrapping the merge as "judgment —
+# review the diff yourself". That judgment still lives here, just moved: the
+# maintainer reviews the handover *before* invoking this, and the script only
+# commits the merge, never the push. The push is owned by `create:release`, so
+# the release commit and its tag leave together and a local merge stays
+# reviewable (and can be reverted with `git revert -m 1`) until then. `--no-ff`
+# keeps the unit of work visible in `git log`.
+#
+# Unlike `create:branch` a stale `main` is fast-forwarded instead of refused:
+# the tree is clean (checked above) and `main` is not the checked-out branch
+# yet, so there is no local state to lose. True divergence (local commits *and*
+# upstream commits) is still refused — that needs a human.
+#
+# On a merge conflict we abort and return to the feature branch, so a failed
+# finish never strands you on a half-merged `main`.
+
+BASE="main"
+PREFIXES="feature fix chore"
+
+git rev-parse --is-inside-work-tree >/dev/null 2>&1 || {
+    echo "error: not inside a git work tree." >&2
+    exit 1
+}
+
+STATE_ROOT=$(git rev-parse --absolute-git-dir)
+for state in MERGE_HEAD rebase-merge rebase-apply CHERRY_PICK_HEAD BISECT_LOG; do
+    [ -e "${STATE_ROOT}/${state}" ] && {
+        echo "error: a '${state}' operation is in progress; finish or abort it first." >&2
+        exit 1
+    }
+done
+
+# `--porcelain` is deliberately stricter than `git diff --quiet`: it also
+# reports untracked files, which would otherwise not be part of the merge and
+# silently outlive the branch deletion.
+DIRTY=$(git status --porcelain)
+if [ -n "${DIRTY}" ]; then
+    echo "error: working tree is not clean:" >&2
+    echo "${DIRTY}" | sed 's/^/         /' >&2
+    exit 1
+fi
+
+START_REF=$(git symbolic-ref --quiet --short HEAD || true)
+if [ -z "${START_REF}" ]; then
+    echo "error: detached HEAD; switch to the branch you want to finish." >&2
+    exit 1
+fi
+
+if [ "${START_REF}" = "${BASE}" ]; then
+    echo "error: already on '${BASE}'; switch to the branch to finish." >&2
+    exit 1
+fi
+
+MATCH=0
+for p in ${PREFIXES}; do
+    case "${START_REF}" in
+    "${p}/"*) MATCH=1 ;;
+    esac
+done
+if [ "${MATCH}" -ne 1 ]; then
+    echo "error: '${START_REF}' must start with one of: ${PREFIXES}." >&2
+    echo "       refusing to merge a branch that is not a unit of work." >&2
+    exit 1
+fi
+
+git show-ref --verify --quiet "refs/heads/${BASE}" || {
+    echo "error: no local '${BASE}' to merge into." >&2
+    exit 1
+}
+
+# Derive the remote rather than hardcoding it: this repo has `origin` (ssh) and
+# `origin_https`, and `main` tracks the latter — `git fetch origin main` would
+# check currency against a ref that is never updated here.
+# `--quiet` echoes the unresolved `main@{upstream}` literal to stdout on
+# failure, so it cannot be paired with a `$(...) || fallback`; the non-quiet
+# form prints nothing on failure and the fallback runs.
+UPSTREAM=$(git rev-parse --abbrev-ref --symbolic-full-name "${BASE}@{upstream}" 2>/dev/null) || UPSTREAM=""
+BEHIND=0
+if [ -n "${UPSTREAM}" ]; then
+    git fetch --quiet "${UPSTREAM%/*}" "${UPSTREAM#*/}" || {
+        echo "error: '${UPSTREAM}' check failed: could not reach '${UPSTREAM%/*}'." >&2
+        echo "       refusing to merge onto a possibly stale '${BASE}'." >&2
+        exit 1
+    }
+    BEHIND=$(git rev-list --count "${BASE}..${UPSTREAM}")
+    AHEAD=$(git rev-list --count "${UPSTREAM}..${BASE}")
+    if [ "${AHEAD}" -ne 0 ] && [ "${BEHIND}" -ne 0 ]; then
+        echo "error: '${BASE}' has diverged from '${UPSTREAM}' (ahead ${AHEAD}, behind ${BEHIND})." >&2
+        echo "       reconcile '${BASE}' with '${UPSTREAM}' before finishing." >&2
+        exit 1
+    fi
+else
+    echo "warning: '${BASE}' has no upstream; freshness against the remote is unchecked." >&2
+fi
+
+echo "Finishing into ${BASE}:"
+git --no-pager log --oneline --no-decorate "${BASE}..${START_REF}" | sed 's/^/  /'
+
+git switch --quiet "${BASE}"
+if [ "${BEHIND}" -ne 0 ]; then
+    echo "Fast-forwarding ${BASE} to ${UPSTREAM} (${BEHIND} commit(s))."
+    git merge --quiet --ff-only "${UPSTREAM}"
+fi
+
+if ! git merge --quiet --no-ff -m ":twisted_rightwards_arrows: Merge ${START_REF} into ${BASE}" "${START_REF}"; then
+    echo "error: merge of '${START_REF}' failed; aborting and returning to it." >&2
+    git merge --abort 2>/dev/null || true
+    git switch --quiet "${START_REF}"
+    exit 1
+fi
+
+echo "Verify: npm run verify"
+if ! npm run --silent verify; then
+    echo "error: 'npm run verify' is red after the merge." >&2
+    echo "       the merge is local and not yet pushed; fix it on '${BASE}' and commit," >&2
+    echo "       then drop the now-merged branch with 'git branch -d ${START_REF}'." >&2
+    exit 1
+fi
+
+git branch --delete "${START_REF}" >/dev/null
+echo "Merged ${START_REF} into ${BASE} and deleted the branch."
+echo "Next: npm run create:release (or git push, for a merge with no release)."