fix(agent-core): preserve Bash stdout after process exit

[barkure]

Related Issue

Resolve #2426

Problem

ProcessBackgroundTask starts consuming stdout and stderr before waiting for the process, but after the process exited it allowed only 250 ms for those streams to end. If stream closure was delayed beyond that grace period, the task disposed the streams and returned a successful Bash result without the late output.

Making KaosProcess.wait() wait for stream closure globally is unsafe: callers are allowed to wait before reading, and BufferedReadable applies backpressure once its 128 KiB prefetch buffer fills. In that usage, waiting for close can deadlock because the paused stream needs a reader before it can close.

What changed

• Keep KaosProcess.wait() tied to direct process exit so wait-before-read remains safe.
• Make ProcessBackgroundTask, which actively consumes both output streams from startup, wait until those streams end before completing and disposing the process.
• Add a task-level regression test for output that arrives after process exit.
• Add a 1 MiB wait-before-read regression test to protect the Kaos process contract from backpressure deadlocks.
• Add a patch changeset for the CLI bundle.

No user documentation update is needed because this restores existing Bash output behavior without adding or changing user-facing configuration.

Verification

• Kaos tests: 295 passed, 22 skipped
• Agent Core background-task tests: 117 passed
• Kaos and Agent Core typechecks
• Kaos and Agent Core builds
• Changeset status: @moonshot-ai/kimi-code patch
• Oxlint on changed files: 0 errors
• git diff --check

The original WSL2 environment was not available for a manual reproduction. The regression tests cover the stream-drain race and the wait-before-read backpressure boundary independently.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 72514bc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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