fix(broker): do not signal stale persisted pids

[sylvesterkaczmarek]

Summary

• stop treating a PID loaded from persisted broker.json as proof of process ownership
• require an explicit ownedPid before teardown may signal a process
• retain the PID fallback for a broker spawned by the current ensureBrokerSession call when startup fails
• add regressions proving SessionEnd does not signal an unrelated live process after PID reuse

Fixes #743.

Validation

• npm test (94/94 passed)
• npm run check-version
• git diff --check

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-07T07:58:27.401394Z	9c854bc	PR opened

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