security: verify PID belongs to claude before SIGKILL (#160)

Add _is_claude_process() that reads /proc/{pid}/comm to confirm the
process is "claude" or "node" before sending SIGKILL, preventing the
kill from hitting an unrelated process if the original claude pid was
recycled by the kernel between cron ticks.

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

scripts/agent_loop.py

@@ -333,6 +333,15 @@ def _agent_alive(state: dict) -> bool:
         return True
 
 
+def _is_claude_process(pid: int) -> bool:
+    """Return True if pid's comm name indicates it is a claude/node process."""
+    try:
+        comm = Path(f"/proc/{pid}/comm").read_text().strip()
+        return comm in ("claude", "node")
+    except OSError:
+        return False
+
+
 def _agent_age_seconds(state: dict) -> float:
     """Seconds elapsed since the agent was launched, from the state file timestamp."""
     try:
@@ -367,11 +376,13 @@ def _git_summary() -> str:
 def _kill_agent(state: dict) -> None:
     """Forcefully stop the running agent."""
     pid = state.get("pid")
-    if pid:
+    if pid and _is_claude_process(pid):
         try:
             os.kill(pid, 9)
         except ProcessLookupError:
             pass
+    elif pid:
+        print(f"WARNING: pid {pid} is not a claude process — skipping kill to avoid hitting recycled PID")
 
 
 # ── subcommands ───────────────────────────────────────────────────────────────

scripts/test_agent_loop.py

@@ -88,13 +88,33 @@ class TestAgentAlive(unittest.TestCase):
         self.assertFalse(agent_loop._agent_alive({"pid": None}))
 
 
+class TestIsClaudeProcess(unittest.TestCase):
+    def test_returns_true_for_claude_comm(self):
+        with patch.object(agent_loop.Path, "read_text", return_value="claude\n"):
+            self.assertTrue(agent_loop._is_claude_process(1234))
+
+    def test_returns_true_for_node_comm(self):
+        with patch.object(agent_loop.Path, "read_text", return_value="node\n"):
+            self.assertTrue(agent_loop._is_claude_process(1234))
+
+    def test_returns_false_for_other_process(self):
+        with patch.object(agent_loop.Path, "read_text", return_value="bash\n"):
+            self.assertFalse(agent_loop._is_claude_process(1234))
+
+    def test_returns_false_when_proc_missing(self):
+        with patch.object(agent_loop.Path, "read_text", side_effect=OSError):
+            self.assertFalse(agent_loop._is_claude_process(1234))
+
+
 class TestKillAgent(unittest.TestCase):
     def test_kill_sends_sigkill(self):
+        with patch("agent_loop._is_claude_process", return_value=True):
             with patch("agent_loop.os.kill") as mock_kill:
                 agent_loop._kill_agent({"pid": 1234})
                 mock_kill.assert_called_once_with(1234, 9)
 
     def test_kill_ignores_missing_process(self):
+        with patch("agent_loop._is_claude_process", return_value=True):
             with patch("agent_loop.os.kill", side_effect=ProcessLookupError):
                 agent_loop._kill_agent({"pid": 1234})  # Should not raise.
 
@@ -103,6 +123,12 @@ class TestKillAgent(unittest.TestCase):
             agent_loop._kill_agent({})
             mock_kill.assert_not_called()
 
+    def test_kill_skips_recycled_pid(self):
+        with patch("agent_loop._is_claude_process", return_value=False):
+            with patch("agent_loop.os.kill") as mock_kill:
+                agent_loop._kill_agent({"pid": 1234})
+                mock_kill.assert_not_called()
+
 
 class TestStartAgent(unittest.TestCase):
     def _make_mock_proc(self, pid=42):