feat: add self-tdd watchdog — continuous test runner CLI

Adds `src/self_tdd/watchdog.py` with a `_run_tests()` function that
shells out to pytest and a `watch` command that polls on a configurable
interval, printing green on recovery and full short-traceback output on
regression. No files are modified and no commits are made automatically.

Usage:
    self-tdd watch            # default 60s interval
    self-tdd watch -i 15      # poll every 15s

Also adds 6 unit tests and wires the `self-tdd` entry point +
`src/self_tdd` wheel include into pyproject.toml.

https://claude.ai/code/session_01DMjQ5qMZ8iHeyix1j3GS7c

src/self_tdd/watchdog.py

@@ -0,0 +1,71 @@
+"""Self-TDD Watchdog — polls pytest on a schedule and reports regressions.
+
+Run in a terminal alongside your normal dev work:
+
+    self-tdd watch
+    self-tdd watch --interval 30
+
+The watchdog runs silently while tests pass. When a regression appears it
+prints the full short-traceback output so you can see exactly what broke.
+No files are modified; no commits are made. Ctrl-C to stop.
+"""
+
+import subprocess
+import sys
+import time
+from datetime import datetime
+from pathlib import Path
+
+import typer
+
+# Project root is three levels up from src/self_tdd/watchdog.py
+PROJECT_ROOT = Path(__file__).parent.parent.parent
+
+app = typer.Typer(help="Self-TDD watchdog — continuous test runner")
+
+
+def _run_tests() -> tuple[bool, str]:
+    """Run the test suite and return (passed, combined_output)."""
+    result = subprocess.run(
+        [sys.executable, "-m", "pytest", "tests/", "-q", "--tb=short"],
+        capture_output=True,
+        text=True,
+        cwd=PROJECT_ROOT,
+        timeout=60,
+    )
+    return result.returncode == 0, (result.stdout + result.stderr).strip()
+
+
+@app.command()
+def watch(
+    interval: int = typer.Option(60, "--interval", "-i", help="Seconds between test runs"),
+) -> None:
+    """Poll pytest continuously and print regressions as they appear."""
+    typer.echo(f"Self-TDD watchdog started — polling every {interval}s. Ctrl-C to stop.")
+    last_passed: bool | None = None
+
+    try:
+        while True:
+            passed, output = _run_tests()
+            stamp = datetime.now().strftime("%H:%M:%S")
+
+            if passed:
+                if last_passed is not True:
+                    typer.secho(f"[{stamp}] All tests passing.", fg=typer.colors.GREEN)
+            else:
+                typer.secho(f"[{stamp}] Regression detected:", fg=typer.colors.RED)
+                typer.echo(output)
+
+            last_passed = passed
+            time.sleep(interval)
+
+    except KeyboardInterrupt:
+        typer.echo("\nWatchdog stopped.")
+
+
+def main() -> None:
+    app()
+
+
+if __name__ == "__main__":
+    main()