build(git-hooks): enforce Conventional Commits via a hook + CI, record history decision

The convention was documented in CONTRIBUTING.md but nothing enforced it, and no
decision was on record for the pre-self-hosting `Phase` history.

- tools/git-hooks/commit-msg — rejects a summary that is not a Conventional
  Commit. tools/git-hooks/lib.sh holds the single rule (types, scope, `!`, and
  the merge/revert/autosquash exemptions) so the hook and CI cannot drift.
- tools/git-hooks/lint-range.sh — lints a commit range with that same rule.
- .forgejo/workflows/commit-lint.yml — runs it over the new commits on every
  push and PR, as the backstop for contributors who have not enabled the hook.
- Fix the pre-commit hook, which pointed at the old build/ludic-fmt path (the
  toolchain moved to bin/) and so silently no-op'd; it now finds bin/ludic-fmt.
- CONTRIBUTING.md — full type/scope table, the one-line enable
  (`git config core.hooksPath tools/git-hooks`), and a **Git history** section
  recording the decision: leave the pushed `Phase`-era history as-is (a rewrite
  is destructive and non-reversible for anyone who cloned); enforce the
  convention going forward; let #33's first release tag double as the clean `v0`
  baseline that brackets the old prefix without touching a commit.

Verified: the hook accepts feat/fix/ci/refactor(!)/merge/revert and rejects
"added regex" / "Fix bug" / "WIP"; lint-range passes recent history and flags
the old `Phase 8b:` commit.

Closes #34

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/git-hooks/commit-msg

@@ -0,0 +1,25 @@
+#!/bin/sh
+# Reject a commit whose summary line is not a Conventional Commit.
+#
+# Enable every hook in this directory at once with:
+#
+#   git config core.hooksPath tools/git-hooks
+#
+# (git then runs the hooks straight from the tree — no per-hook symlink, and new
+# hooks activate as soon as they land on your branch.)
+set -e
+MSG_FILE="$1"
+
+# The header is the first line that is neither blank nor a comment (git strips
+# '#' lines itself, but a commit-msg hook runs before that, so skip them here).
+HEADER=$(grep -vE '^[[:space:]]*#' "$MSG_FILE" | sed '/^[[:space:]]*$/d' | head -n1)
+
+. "$(dirname "$0")/lib.sh"
+
+if conventional_ok "$HEADER"; then
+  exit 0
+fi
+
+echo "commit-msg hook: rejected." >&2
+conventional_help "$HEADER" >&2
+exit 1

tools/git-hooks/lib.sh

@@ -0,0 +1,34 @@
+# lib.sh — shared Conventional Commits validation, sourced by the commit-msg
+# hook and by the CI commit-lint job so the rule lives in exactly one place.
+# POSIX sh; no bashisms.
+
+# The allowed commit types (Conventional Commits + the ones this repo uses).
+CONVENTIONAL_TYPES='feat|fix|docs|style|refactor|perf|test|build|ci|chore|revert'
+
+# conventional_ok <header-line> -> 0 if it conforms, non-zero otherwise.
+# Accepts `type(scope): summary`, an optional `!` for a breaking change, and the
+# housekeeping headers git itself writes (merge/revert) or autosquash markers.
+conventional_ok() {
+  header="$1"
+  [ -n "$header" ] || return 1
+  case "$header" in
+    "Merge "*|"Revert "*|"fixup! "*|"squash! "*|"amend! "*) return 0 ;;
+  esac
+  printf '%s\n' "$header" \
+    | grep -qE "^($CONVENTIONAL_TYPES)(\([a-z0-9][a-z0-9,._/ -]*\))?(!)?: .+"
+}
+
+# conventional_help <header-line> — print a short, actionable explanation.
+conventional_help() {
+  echo "  commit summary is not a Conventional Commit:"
+  echo "      ${1:-<empty>}"
+  echo
+  echo "  expected:  type(scope): summary        (scope is optional)"
+  echo "  types:     feat fix docs style refactor perf test build ci chore revert"
+  echo "  examples:  feat(stdlib): add Regex.* namespace"
+  echo "             fix(emit): stop fixed compares widening the int literal"
+  echo "             ci: cache the clang toolchain between runs"
+  echo "             refactor(stdlib)!: drop the deprecated System.env alias"
+  echo
+  echo "  see https://www.conventionalcommits.org"
+}

tools/git-hooks/lint-range.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Lint every commit summary in a git range against the Conventional Commits rule.
+# Used by the commit-lint CI job; usable by hand too:
+#
+#   tools/git-hooks/lint-range.sh origin/main..HEAD
+#
+# With no argument it lints just the tip commit. Reports every commit, then exits
+# non-zero if any failed.
+set -e
+RANGE="${1:-HEAD~1..HEAD}"
+
+. "$(dirname "$0")/lib.sh"
+
+# Collect subjects into a file so the loop runs in this shell (a pipe would put
+# it in a subshell and lose the verdict). Merge commits are exempt by the rule
+# and skipped here so a merge subject never trips the lint.
+tmp=$(mktemp)
+git log --no-merges --format='%s' "$RANGE" > "$tmp"
+
+fail=0
+while IFS= read -r subject; do
+  [ -n "$subject" ] || continue
+  if conventional_ok "$subject"; then
+    echo "  ok    $subject"
+  else
+    echo "  FAIL  $subject"
+    conventional_help "$subject"
+    fail=1
+  fi
+done < "$tmp"
+rm -f "$tmp"
+
+[ "$fail" -eq 0 ]

tools/git-hooks/pre-commit

@@ -2,12 +2,14 @@
 # Reject a commit that contains unformatted Ludic — in source or in the ```ludic
 # fences of a Markdown file.
 #
-#   ln -sf ../../tools/git-hooks/pre-commit .git/hooks/pre-commit
+# Enable this and the commit-msg hook together with:
+#
+#   git config core.hooksPath tools/git-hooks
 #
 # Checks only the files being committed, so it stays fast on a big tree.
 set -e
 ROOT=$(git rev-parse --show-toplevel)
-FMT="$ROOT/build/ludic-fmt"
+FMT="$ROOT/bin/ludic-fmt"
 
 [ -x "$FMT" ] || exit 0        # toolchain not built: do not block the commit