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

tests/test_watchdog.py

@@ -0,0 +1,54 @@
+from unittest.mock import MagicMock, patch
+
+from self_tdd.watchdog import _run_tests
+
+
+def _mock_result(returncode: int, stdout: str = "", stderr: str = "") -> MagicMock:
+    m = MagicMock()
+    m.returncode = returncode
+    m.stdout = stdout
+    m.stderr = stderr
+    return m
+
+
+def test_run_tests_returns_true_when_suite_passes():
+    with patch("self_tdd.watchdog.subprocess.run", return_value=_mock_result(0, "5 passed")):
+        passed, _ = _run_tests()
+    assert passed is True
+
+
+def test_run_tests_returns_false_when_suite_fails():
+    with patch("self_tdd.watchdog.subprocess.run", return_value=_mock_result(1, "1 failed")):
+        passed, _ = _run_tests()
+    assert passed is False
+
+
+def test_run_tests_output_includes_stdout():
+    with patch("self_tdd.watchdog.subprocess.run", return_value=_mock_result(0, stdout="5 passed")):
+        _, output = _run_tests()
+    assert "5 passed" in output
+
+
+def test_run_tests_output_combines_stdout_and_stderr():
+    with patch(
+        "self_tdd.watchdog.subprocess.run",
+        return_value=_mock_result(1, stdout="FAILED test_foo", stderr="ImportError: no module named bar"),
+    ):
+        _, output = _run_tests()
+    assert "FAILED test_foo" in output
+    assert "ImportError" in output
+
+
+def test_run_tests_invokes_pytest_with_correct_flags():
+    with patch("self_tdd.watchdog.subprocess.run", return_value=_mock_result(0)) as mock_run:
+        _run_tests()
+    cmd = mock_run.call_args[0][0]
+    assert "pytest" in cmd
+    assert "tests/" in cmd
+    assert "--tb=short" in cmd
+
+
+def test_run_tests_uses_60s_timeout():
+    with patch("self_tdd.watchdog.subprocess.run", return_value=_mock_result(0)) as mock_run:
+        _run_tests()
+    assert mock_run.call_args.kwargs["timeout"] == 60