fix(agent-core): tolerate unreadable .git probes on UNC workspaces

[djfch]

Related Issue

Resolve #2540

Problem

See linked issue. On Windows, workspaces opened via a UNC path (network share, e.g. a NAS) that are not git repositories cannot send any chat message: the project-root walk stats \\server\.git past the share root, Node/libuv reports UNKNOWN instead of ENOENT, and the probe helper rethrew it, failing the whole chat preflight.

What changed

• The .git marker probe in the MCP config loader's project-root walk now treats any stat failure as "not found" instead of rethrowing non-ENOENT/ENOTDIR errors — a probe for an optional marker file should never throw. This matches the existing probe helpers elsewhere in the repo (e.g. workspace-local config, profile context) and also covers EPERM-style failures, not just UNKNOWN.
• Removed the now-unused error-code helper from the same file.
• Added regression tests that simulate UNKNOWN and EPERM probe failures and assert the walk falls back to the starting directory (platform-independent via a partial node:fs/promises mock).

The sibling probe copies in the agent-file root resolvers (both engines) were left untouched: their callers already catch and downgrade probe errors to warnings, so they never had this crash.

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.

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🦋 Changeset detected

Latest commit: 7e912cf

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

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@moonshot-ai/kimi-code	Patch

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