fix(os): normalize Windows drive-root paths for fs.watch

[ktz03]

Summary

• On Windows, starting from a drive root CWD such as D: can produce a POSIX-ish watch path /D:.
• fs.watch('/D:') then fails with ENOENT: no such file or directory, watch '/D:'.
• Normalize drive-only roots to D:\ before creating native/chokidar watchers.
• Add a regression test for /D: and E:.

Related: MoonshotAI/kimi-cli#2600

Testing plan

• Added unit test that watchNative receives D:\\ when given /D: / E: on win32.
• Manual: set PowerShell start directory to D:\ (drive root), launch Kimi Code, confirm no watch '/D:' crash.

[changeset-bot]

⚠️ No Changeset found

Latest commit: dd4de95

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Move the implementation comment to the file header

Move this explanation into the top-of-file module header or make the helper self-explanatory without it. The package explicitly prohibits comments beside functions or statements, and these newly added lines narrate the normalization implementation inside the module body.

AGENTS.md reference: packages/agent-core-v2/AGENTS.md:L36-L38

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[ktz03]

Addressed Codex P1: moved the Windows drive-root explanation into the file header and removed the inline comments per packages/agent-core-v2/AGENTS.md comment conventions.

Re: changeset — this is a bugfix; no package version bump intended, so no changeset added.