[WRONG BRANCH] fix(update): avoid terminating reused child PIDs

[luvs01]

Motivation

• Prevent a restart retry from killing an unrelated process when a previously spawned detached start has already exited and the OS has reused its numeric PID.

Description

• Stop treating a retained ChildProcess as owned solely by its numeric PID: add a killSpawnAttempt guard that checks child.exitCode and child.signalCode before calling killProxy.
• Clear the retained lastChild when the detached start emits exit by attaching an exit handler to the spawned child, and use the now() seam for deterministic health deadlines.
• Add test seams spawnDetachedStartFn and killProxyFn so unit tests can inject deterministic child behavior without changing production flows.
• Add a regression test restart retries never kill a PID retained from an exited pinned start that simulates an exited child (numeric PID still appears) and asserts no kill attempts occur across retries.

Testing

• Ran the focused update tests with the repository Bun binary: ./node_modules/.bin/bun test tests/update-job.test.ts, which passed (45 tests).
• Ran typechecking with ./node_modules/.bin/bun run typecheck, which passed.
• Ran privacy scan with ./node_modules/.bin/bun run privacy:scan, which passed.
• Ran the full test suite (./node_modules/.bin/bun run test); the focused restart/update tests passed, while an unrelated integration test (CLI OAuth live-update credential preservation) timed out in that run (timeout unrelated to this change).

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 2468502 and 28bb1fb.

📒 Files selected for processing (2)

• src/update/job.ts
• tests/update-job.test.ts

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[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• wrong target branch (main); retarget to dev.

What to do

• Retarget this PR to dev — all contributions go to dev.

Its title has been prefixed with [WRONG BRANCH].
This pull request was already a draft. Its draft status will be preserved after every issue above is resolved.