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/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