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fix(main): use exitCode/signalCode for liveness instead of .killed

Addresses Copilot review feedback on PR #2217.

`ChildProcess.killed` is only set after a successful `kill()` call - it
reflects "we sent a signal", not "the process has exited". After a
graceful SIGTERM the flag becomes true while the child is still running,
which caused the very fix this PR is for to short-circuit: the
parent-exit handler's `stopToolhive({ force: true })` returned early on
`toolhiveProcess.killed` and never escalated to SIGKILL. The mock test
deliberately did not auto-set `.killed`, so the regression was masked.

- isToolhiveRunning() now checks exitCode/signalCode for true liveness
- stopToolhive() early return uses the same liveness check
- exit handler clears the pending killTimer so a quick graceful exit
  does not leave a 2s setTimeout holding the event loop open
- MockProcess gains exitCode/signalCode (null defaults) to match the
  real ChildProcess API; regression test now sets mockProcess.killed
  after the first stop so the escalation path mirrors production

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>