fix(browser): stop treating a missing cmdline as proof the daemon exited

_terminate_owned_daemon() read /proc/<pid>/cmdline and, on FileNotFoundError,
unlinked the pid file on the stated assumption that "the daemon may have
exited". Off Linux that file is always missing, so the branch always fired:
the pid file of a live daemon was deleted and the daemon itself never killed.
_owned_daemon_exists() swallowed the same error and therefore always returned
False, which is precisely the state its own docstring warns about, since a
close against an unrecognised session can bootstrap a fresh daemon and wait on
its browser indefinitely.

Demonstrated on macOS before the change: a pid file holding a live pid is
removed by _terminate_owned_daemon() and _owned_daemon_exists() reports False.
After it, the file survives and the daemon is reported present.

_process_command_line() now returns None for "this host cannot tell" and
_process_is_alive() answers the separate question of whether the pid exists.
Without procfs we decline to kill a process we cannot confirm is ours, and we
only forget a pid file once the pid is genuinely gone. Linux behaviour is
unchanged: the command-line identity check still gates both paths.

src/agent_tools/web_tools.py

@@ -125,6 +125,37 @@ def _browser_pid_file_candidates(
 # host that has no procfs, and on one that does.
 _PROC_ROOT = Path("/proc")
 
+
+def _process_command_line(pid: int) -> str | None:
+    """Command line of a running process, or ``None`` when it cannot be read.
+
+    ``None`` means "this host cannot tell", not "the process is gone". Off
+    Linux there is no procfs to read a command line from, so callers must not
+    treat it as proof that the process exited.
+    """
+
+    try:
+        return (_PROC_ROOT / str(pid) / "cmdline").read_bytes().replace(
+            b"\0", b" "
+        ).decode("utf-8", errors="replace")
+    except (OSError, UnicodeError):
+        return None
+
+
+def _process_is_alive(pid: int) -> bool:
+    """Whether a pid currently exists. Signal 0 checks without delivering."""
+
+    try:
+        os.kill(pid, 0)
+    except ProcessLookupError:
+        return False
+    except PermissionError:
+        # Alive, owned by somebody else.
+        return True
+    except OSError:
+        return False
+    return True
+
 _SCHOLARLY_METADATA_CUE_RE = re.compile(
     r"\b(?:accept(?:ed|ance)?|publish(?:ed|ing|cation)?|venue|conference|"
     r"journal|proceedings|doi)\b",
@@ -2359,16 +2390,18 @@ class PrivateBrowserTool:
         for pid_file in pid_files:
             try:
                 pid = int(pid_file.read_text().strip())
-                command_line = (Path("/proc") / str(pid) / "cmdline").read_bytes().replace(
-                    b"\0", b" "
-                ).decode("utf-8", errors="replace")
-            except FileNotFoundError:
-                # The daemon may have exited between writing its pid file and
-                # this cleanup pass.  The exact file is still ours to remove.
-                with contextlib.suppress(FileNotFoundError, PermissionError, OSError):
-                    pid_file.unlink()
+            except (OSError, ValueError):
                 continue
-            except (OSError, UnicodeError, ValueError):
+            command_line = _process_command_line(pid)
+            if command_line is None:
+                # Either the daemon exited between writing its pid file and
+                # this pass, or this host has no procfs to ask. Only the first
+                # justifies forgetting the pid file. Without procfs we cannot
+                # confirm the process is ours, so we neither kill it nor drop
+                # the record that would let a later pass find it.
+                if not _process_is_alive(pid):
+                    with contextlib.suppress(FileNotFoundError, PermissionError, OSError):
+                        pid_file.unlink()
                 continue
             if "agent-browser" in command_line:
                 with contextlib.suppress(ProcessLookupError, PermissionError, OSError):
@@ -2395,10 +2428,17 @@ class PrivateBrowserTool:
         for pid_file in _browser_pid_file_candidates(runtime_dir, namespace, session_id):
             try:
                 pid = int(pid_file.read_text().strip())
-                command_line = (Path("/proc") / str(pid) / "cmdline").read_bytes().replace(
-                    b"\0", b" "
-                ).decode("utf-8", errors="replace")
-            except (FileNotFoundError, OSError, UnicodeError, ValueError):
+            except (OSError, ValueError):
+                continue
+            command_line = _process_command_line(pid)
+            if command_line is None:
+                # Without procfs we can only tell that something with this pid
+                # is alive, not that it is agent-browser. The pid file is our
+                # own namespaced one, so treat a live pid as a match: answering
+                # "no daemon" here is what lets `close` bootstrap a fresh one
+                # and wait on its browser forever.
+                if _process_is_alive(pid):
+                    return True
                 continue
             if "agent-browser" in command_line:
                 return True