fix(cookbook): skip the pid sweep when the host has no procfs

`_cookbook_kill_session` kills the tmux session, then sweeps /proc for
model servers that survived the SIGHUP. The sweep had no guard, so on
macOS and Windows `os.listdir("/proc")` raised FileNotFoundError after
the kill had already succeeded. The function's outer except turned that
into `{"error": "...No such file or directory: '/proc'", "exit_code": 1}`
and skipped the state write that marks the session stopped — the agent
is told a stop failed that actually worked.

Guard the sweep with a procfs check, the way `_scan_running_model_processes`
already does a few hundred lines up. The root and the check now live in
`core/platform_compat`, which is where OS differences belong and which
makes both branches patchable from a test on either kind of host.

Adds a class-level guard test: the third instance of this defect, and two
of the three were found by reading source rather than by a test.

core/platform_compat.py

@@ -36,6 +36,19 @@ IS_APPLE_SILICON = (
 )
 
 
+# ── procfs ──────────────────────────────────────────────────────────────────
+# Linux exposes one directory per pid under /proc; macOS and Windows have no
+# procfs at all. Any code that walks it must skip the walk rather than raise.
+# Kept as a module attribute so both branches stay testable on either kind of
+# host.
+PROC_ROOT = Path("/proc")
+
+
+def has_procfs() -> bool:
+    """True when the host exposes a procfs pid tree that can be scanned."""
+    return PROC_ROOT.is_dir()
+
+
 # ── File permissions ────────────────────────────────────────────────────────
 def safe_chmod(path, mode: int) -> bool:
     """``os.chmod`` that is a harmless no-op on Windows.