fix(core): resolve spawn on exit not close to avoid bg-child hang

[divitkashyap]

Issue for this PR

Closes #20902

Type of change

• Bug fix

What does this PR do?

The cross-spawn spawner (packages/core/src/cross-spawn-spawner.ts) only settled its exit Deferred on the Node close event. close waits for all stdio descriptors to drain — so if the spawned command forks a background grandchild that inherits stdout/stderr (e.g. cmd &, daemonized servers, the Gradle daemon, long-lived stdio MCP servers), close never fires and the shell tool hangs until the configured timeout (~2 minutes).

The fix is to settle on whichever of exit or close fires first. exit fires when the direct child exits regardless of inherited-fd lifetime — which is what callers of handle.exitCode actually want. close is kept as a fallback in case exit never fires for some reason. The exit-code/signal payload comes from whichever event ran first, which matches what Node passes to both.

How did you verify your code works?

• Added a regression test in packages/opencode/test/tool/shell.test.ts that runs sleep 30 & disown; echo backgrounded through the shell tool and asserts it returns in well under the timeout. Without the fix it hangs the full 15s test timeout; with the fix it returns in ~1s.
• Ran the full packages/opencode/test/tool/shell.test.ts suite (24 tests pass) and packages/core/test/effect/cross-spawn-spawner.test.ts (24 tests pass) locally.
• Did not run the full repo typecheck — that pipeline is slow and the change is isolated to one well-typed function.

Checklist

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

[divitkashyap]

@thdxr would appreciate a look when you have a moment — small, contained fix with a regression test. Happy to iterate on the approach if you'd prefer something different.