fix(opencode): add SIGKILL fallback to Process.stop()

[beowulfof]

Issue for this PR

Closes #36558

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Process.stop() in packages/opencode/src/util/process.ts only sent proc.kill() (SIGTERM) with no timeout or SIGKILL fallback. When an LSP server ignores SIGTERM (e.g. rust-analyzer, clangd), the process remains as a zombie — no escalation to SIGKILL ever happens.

The abort handler in Process.spawn() (same file, line 74) already had the correct pattern: SIGTERM → 5s timeout → SIGKILL. stop() was simply missing this escalation chain.

The fix wraps the existing proc.kill("SIGTERM") with a race between the process exit event and a 5-second timer that escalates to SIGKILL. The SDK copy (packages/sdk/js/src/process.ts) is updated to stay in sync.

How did you verify your code works?

• bun typecheck: 30/30 tasks, all pass
• bun test test/util/process.test.ts: 10/10 tests pass

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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