fix(agent-core): bound Git diff output while reading

[morluto]

Related Issue

Resolve #2169

Problem

The Git services applied their 1 MiB file-diff limit only after collecting complete subprocess stdout. Large diffs could therefore create memory spikes proportional to the full diff even though callers received at most 1 MiB.

The old limit also measured JavaScript string length rather than UTF-8 bytes, allowing multibyte responses to exceed the intended byte ceiling.

A representative 24 MiB modified file increased peak RSS to 331 MiB before this change and 231 MiB afterward. The measurement includes Git, fixture data, decoding, and V8 overhead; the code-level guarantee is that the retained diff response is now bounded rather than scaling with complete stdout.

What changed

• Bound file-diff stdout while consuming it in both the legacy and v2 engines.
• Retain at most 1 MiB by UTF-8 byte count.
• Stop at a valid Unicode boundary.
• Record whether later output was discarded.
• Continue draining stdout after reaching the limit so Git cannot block on a full pipe.

Other Git commands, stderr collection, timeout behavior, and exit-code handling are unchanged.

Regression tests exercise both services through their existing public boundaries with a 1.2 MiB multibyte diff. They fail on the previous implementation because it returns truncated: false and exceeds the byte limit.

Validation:

• Legacy Git service tests: 7 passed
• V2 Git service tests: 8 passed
• Both package typechecks
• V2 domain lint and root lint
• git diff --check
• Changeset validation

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, and included a patch changeset.
• This restores the existing diff-size contract and needs no documentation update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 676a072

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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