docs: clarify subagent delegation boundaries

[starSumi]

Related Issue

No linked issue. This is a documentation-only clarification for current subagent, MCP, and ACP behavior.

Problem

The docs describe subagents, MCP forwarding, and kimi acp separately, but they do not explicitly say where the runtime boundary is. Readers can reasonably wonder whether Kimi subagents can directly dispatch external agent runtimes such as Claude Code, Codex, or another ACP agent.

What changed

• Clarified that Kimi subagents are Kimi Code runtime agents using the current session model/provider and a Kimi subagent profile.
• Clarified that MCP tools exposed inside a subagent remain tool calls, even if they wrap another local CLI or agent.
• Clarified that kimi acp makes Kimi Code the ACP agent process driven by an IDE/client, not an ACP client that dispatches prompts to other ACP agents.
• Mirrored the updates in both English and Chinese docs.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• Docs-only change; no tests added. Validated with the docs build.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

Validation:

• git diff --check
• pnpm -C docs install --ignore-workspace
• pnpm -C docs run build (passes; VitePress reports the existing chunk-size warning)

Changeset: not added because this is a docs-only PR.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 79fe6cc

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