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