MCP stdio servers are spawned via the SDK's stdio_client, which on
Linux uses start_new_session=True (setsid).  When a cron job is
cancelled mid-way (timeout, agent finish, exception), the subprocess
often escapes the SDK's teardown and survives as a session leader.
Because setsid() detaches the child from the gateway's process group
/ cgroup tree, systemd does not reap it on service restart either —
so every cron tick that touches an MCP tool leaks a dangling server
process.

Fix:

* tools/mcp_tool.py — _run_stdio now wraps the whole stdio+session
  context in try/finally.  On any exit path (clean, exception,
  cancellation), PIDs still alive are moved from the active
  _stdio_pids set into a new _orphan_stdio_pids set.  Orphan
  detection is done via os.kill(pid, 0) — a cheap liveness probe
  that never signals the target.

* tools/mcp_tool.py — _kill_orphaned_mcp_children gains an
  include_active=False flag.  Default behaviour now only reaps the
  orphan set so concurrent sessions (other parallel cron jobs or
  live user chats) are never disrupted.  The existing shutdown path
  passes include_active=True to keep the previous "kill everything"
  semantics after the MCP loop is stopped.

* cron/scheduler.py — the cleanup hook is moved from run_job()'s
  finally (which would race with parallel siblings after #13021)
  into tick() after the ThreadPoolExecutor has joined every future.
  At that point there are no in-flight sessions from this tick, so
  sweeping the orphan set is always safe.

Net effect: zero regression for healthy sessions, and orphan MCP
servers no longer accumulate between gateway restarts.

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