chore: PR title lint + release-drafter (#12) (#52)

Adds the second half of #12 (release-drafter); pr-title.yml itself shipped
in #10's bundle. Port .github/workflows/release-drafter.yml + the categories /
autolabeler / version-resolver config in .github/release-drafter.yml verbatim
from Teller. Pin release-drafter@v6 to commit SHA 6a93d829....

release-drafter is exempt from required-status-checks (workflow runs on push
to main + PR label events; never on every PR). Already in EXEMPT_WORKFLOWS in
check_required_contexts.py.

Closes #12

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: constk

.github/release-drafter.yml

@@ -0,0 +1,82 @@
+# Release-drafter configuration.
+#
+# Drafts release notes by reading the conventional-commit prefix of each
+# merged PR title (squash-merge subject) and grouping into the categories
+# below. Categories map 1:1 to the [tool.commitizen].customize.schema_pattern
+# types enforced by `Lint PR title` CI; see check_commit_types.py.
+
+name-template: "v$RESOLVED_VERSION"
+tag-template: "v$RESOLVED_VERSION"
+
+categories:
+  - title: "Features"
+    labels:
+      - "feat"
+  - title: "Fixes"
+    labels:
+      - "fix"
+  - title: "Housekeeping"
+    labels:
+      - "chore"
+      - "docs"
+      - "refactor"
+      - "test"
+      - "release"
+
+# Match conventional-commit prefixes against PR titles regardless of label —
+# release-drafter assigns the synthetic label so the categories above resolve
+# even when the PR doesn't carry a manual label.
+autolabeler:
+  - label: "feat"
+    title:
+      - "/^feat(\\(.+\\))?:/i"
+  - label: "fix"
+    title:
+      - "/^fix(\\(.+\\))?:/i"
+  - label: "chore"
+    title:
+      - "/^chore(\\(.+\\))?:/i"
+  - label: "docs"
+    title:
+      - "/^docs(\\(.+\\))?:/i"
+  - label: "refactor"
+    title:
+      - "/^refactor(\\(.+\\))?:/i"
+  - label: "test"
+    title:
+      - "/^test(\\(.+\\))?:/i"
+  - label: "release"
+    title:
+      - "/^release(\\(.+\\))?:/i"
+
+# SemVer resolver:
+#   - any `feat` PR since the last release → minor bump
+#   - any `fix` PR with no `feat` → patch bump
+#   - everything else (chore/docs/refactor/test/release-only) → patch bump
+# `major` is not auto-resolved — major bumps are deliberate human calls.
+version-resolver:
+  minor:
+    labels:
+      - "feat"
+  patch:
+    labels:
+      - "fix"
+      - "chore"
+      - "docs"
+      - "refactor"
+      - "test"
+      - "release"
+  default: patch
+
+template: |
+  ## What's changed in v$RESOLVED_VERSION
+
+  $CHANGES
+
+  **Full changelog:** https://github.com/$OWNER/$REPOSITORY/compare/$PREVIOUS_TAG...v$RESOLVED_VERSION
+
+change-template: "- $TITLE ([#$NUMBER]($URL))"
+change-title-escapes: '\<*_&'  # escape markdown control chars in PR titles
+
+exclude-labels:
+  - "skip-changelog"

.github/workflows/release-drafter.yml

@@ -0,0 +1,31 @@
+name: Release Drafter
+
+# Updates a draft GitHub Release on every push to `main` and on PR
+# label / open / reopen events. The drafted release body becomes the
+# `release.yml` workflow's input on tag push (see release.yml).
+#
+# Single-source-of-truth: the categories + version-resolver in
+# .github/release-drafter.yml mirror the conventional-commit prefixes
+# enforced by check_commit_types.py and the `Lint PR title` job.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    types: [opened, reopened, synchronize, labeled, unlabeled]
+    branches: [main]
+
+permissions:
+  contents: write       # create/update draft releases
+  pull-requests: read
+
+jobs:
+  draft:
+    runs-on: ubuntu-latest
+    name: Draft next release
+    steps:
+      - uses: release-drafter/release-drafter@6a93d829887aa2e0748befe2e808c66c0ec6e4c7  # v6
+        with:
+          config-name: release-drafter.yml
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}