fix: clean up background tasks on session exit

[7Sageer]

Related Issue

No linked issue; this is a focused fix for benchmark/headless sessions that should not leave Kimi-managed background tasks running after the session exits. The problem is explained below.

Problem

Headless sessions can start long-running Bash commands as Kimi-managed background tasks. The previous default kept those tasks alive when the session closed, which is surprising in benchmark and automation environments: the main kimi -p process can exit while its managed background work continues to hold resources and delay the outer runner.

What changed

• Change the session-close default so Kimi-managed background tasks are stopped unless keep-alive is explicitly enabled.
• Preserve the opt-in escape hatch with background.keepAliveOnExit = true or KIMI_CODE_BACKGROUND_KEEP_ALIVE_ON_EXIT=true.
• Update lifecycle coverage for the new default and explicit keep-alive opt-in.
• Update English and Chinese configuration docs, plus a major changeset for the behavior change.

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: 2b70029

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

This PR includes changesets to release 2 packages

Name	Type
@moonshot-ai/agent-core	Patch
@moonshot-ai/kimi-code	Patch

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@2b70029

npx https://pkg.pr.new/@moonshot-ai/kimi-code@2b70029

commit: 2b70029