fix: preserve config symlink writes

[ktwu01]

Related Issue

Resolve #751

Problem

When ~/.kimi-code/config.toml is a symlink, saving config updates replaces the symlink itself with a regular file. That breaks user workflows that sync the real config file through another location.

What changed

Config writes now detect a symlinked config path and atomically write to the resolved target file instead of replacing the symlink entry. The generic atomic write helper keeps its existing replacement semantics for other stores, and the new test covers preserving the symlink while updating the target file content.

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: 6509e7e

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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