Run the regression suite automatically (CI + SessionStart hook) (#3)

Make the stdlib regression suite (tests/test_fixtures.py) run automatically
in both CI and Claude Code on the web, so the classifier->plumbing contract
is enforced rather than only checked by hand.

- .github/workflows/tests.yml: runs the suite on every push to main and on
  every PR. No install step — the project is standard-library only.
- .claude/hooks/session-start.sh + .claude/settings.json: a SessionStart
  hook for web sessions. There are no dependencies to install, so instead of
  an install step it runs the suite as a fast, side-effect-free smoke check
  so a session starts knowing the fixtures are green.


Claude-Session: https://claude.ai/code/session_01SAn8aZTHDrPaKj8BoryGFr

Co-authored-by: Claude <noreply@anthropic.com>

Author: brianleach

.claude/hooks/session-start.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# SessionStart hook for Claude Code on the web.
+#
+# This project is standard-library only — there are no dependencies to install.
+# So instead of an install step, we run the offline regression suite as a fast
+# smoke check (it is stdlib-only and side-effect free), so the session starts
+# knowing the classifier->plumbing contract is green.
+set -euo pipefail
+
+cd "${CLAUDE_PROJECT_DIR:-.}"
+
+if ! command -v python3 >/dev/null 2>&1; then
+  echo "session-start: python3 not found; skipping regression smoke check." >&2
+  exit 0
+fi
+
+python3 tests/test_fixtures.py

.claude/settings.json

@@ -0,0 +1,14 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/session-start.sh"
+          }
+        ]
+      }
+    ]
+  }
+}

.github/workflows/tests.yml

@@ -0,0 +1,18 @@
+name: tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  fixtures:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.x"
+      # No dependencies to install — the project is standard-library only.
+      - name: Run regression suite
+        run: python3 tests/test_fixtures.py