fix(runtime): never delete a browser profile lock a live process still holds

[volod-vana]

Summary

launchPersistentContext removed SingletonLock unconditionally before every launch, on the assumption that the lock is always debris from a crash. That assumption is only half true:

• after a crash, clearing it is correct and stays unchanged here
• while another run is live, clearing it lets a second Chromium onto the same profile, which is exactly the corruption the lock exists to prevent

And when the removal does not take, the person sees a raw ProcessSingleton call log blaming Chromium, with no hint that another vana connect is the thing holding the profile.

This reads the lock's symlink target (<hostname>-<pid>, where the hostname itself may contain dashes, so only the final segment is the pid) and checks whether that pid is alive. Live holder: stop with a message naming the pid and how to clear it. Dead or unparseable: remove as stale, as before.

How this showed up

A vana connect linkedin --json --ipc run opened a headed Chrome for manual sign-in and parked at Complete any remaining verification, then click "Done". Nobody clicked it. Ninety-five minutes later it still held the profile, and every retry failed with the Chromium dump:

Failed to create a ProcessSingleton for your profile directory.
... Failed to create /Users/.../browser-profiles/linkedin-playwright/SingletonLock: File exists (17)

After this change that reads:

Another Vana browser session (pid 80496) is already using the profile at
/Users/.../browser-profiles/linkedin-playwright. That is usually an earlier
`vana connect` still waiting for you to finish signing in. Finish that
sign-in, or stop it with `kill 80496`, then try again.

Follow-up worth tracking separately

An interactive prompt under --json --ipc has no visible UI and no timeout, so it can block a profile indefinitely. Detecting the lock turns that into a clear message, but the underlying "interactive prompt in a non-interactive run" case still deserves a timeout.

Test plan

• pnpm validate green (376 tests)
• new unit tests: live pid, dead pid, missing lock, pid-less target
• verified against real filesystem symlinks, not just mocks: live pid returns the pid, unreachable pid and absent lock both return null
• cleared the real hung run on this machine and confirmed the profile and lock files were released

[github-actions]

🎉 This PR is included in version 0.24.0 🎉

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