feat(daemon): add PID file management module

Atomic write-then-rename, stale-PID detection via os.kill on POSIX and
ctypes.OpenProcess on Windows, context manager for cleanup on exit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_daemon_pid.py

@@ -0,0 +1,103 @@
+"""Unit tests for daemon PID file management."""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+import pytest
+
+from pyra.daemon.pid import PidFile, PidFileError, resolve_pid_path
+
+
+def test_write_creates_file(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    p.write()
+    assert (tmp_path / "daemon.pid").exists()
+    assert int((tmp_path / "daemon.pid").read_text().strip()) == os.getpid()
+
+
+def test_read_returns_none_when_absent(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    assert p.read() is None
+
+
+def test_read_returns_pid_when_present(tmp_path: Path) -> None:
+    pid_file = tmp_path / "daemon.pid"
+    pid_file.write_text("12345")
+    p = PidFile(pid_file)
+    assert p.read() == 12345
+
+
+def test_read_returns_none_on_bad_content(tmp_path: Path) -> None:
+    pid_file = tmp_path / "daemon.pid"
+    pid_file.write_text("not-a-number")
+    p = PidFile(pid_file)
+    assert p.read() is None
+
+
+def test_is_stale_false_for_self(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    p.write()
+    assert not p.is_stale()
+
+
+def test_is_stale_true_for_dead_pid(tmp_path: Path) -> None:
+    pid_file = tmp_path / "daemon.pid"
+    pid_file.write_text("999999999")  # unrealistically large PID
+    p = PidFile(pid_file)
+    assert p.is_stale()
+
+
+def test_is_stale_false_when_file_absent(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    assert not p.is_stale()
+
+
+def test_remove_deletes_file(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    p.write()
+    p.remove()
+    assert not (tmp_path / "daemon.pid").exists()
+
+
+def test_remove_is_idempotent(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    p.remove()  # must not raise
+
+
+def test_context_manager_writes_and_removes(tmp_path: Path) -> None:
+    pid_file = tmp_path / "daemon.pid"
+    p = PidFile(pid_file)
+    with p:
+        assert pid_file.exists()
+        assert int(pid_file.read_text().strip()) == os.getpid()
+    assert not pid_file.exists()
+
+
+def test_write_raises_when_live_pid_exists(tmp_path: Path) -> None:
+    p = PidFile(tmp_path / "daemon.pid")
+    p.write()  # writes self PID (which is alive)
+    p2 = PidFile(tmp_path / "daemon.pid")
+    with pytest.raises(PidFileError):
+        p2.write()
+
+
+def test_write_succeeds_over_stale_pid(tmp_path: Path) -> None:
+    pid_file = tmp_path / "daemon.pid"
+    pid_file.write_text("999999999")  # stale
+    p = PidFile(pid_file)
+    p.write()  # should not raise
+    assert int(pid_file.read_text().strip()) == os.getpid()
+
+
+def test_resolve_pid_path_expands_tilde() -> None:
+    result = resolve_pid_path("~/.pyra/daemon.pid")
+    assert not str(result).startswith("~")
+    assert result.is_absolute()
+
+
+def test_resolve_pid_path_absolute_unchanged(tmp_path: Path) -> None:
+    path = tmp_path / "daemon.pid"
+    result = resolve_pid_path(str(path))
+    assert result == path