fix(ckg): accept file path for ckg tool and clarify path parameter

[nankingjing]

Summary

The ckg tool's path parameter description was ambiguous ("The path to the codebase."), leading some models (e.g. Doubao, see #354) to pass a file path instead of the codebase directory. This caused a hard failure (Codebase path X is not a directory) and the CKG was never initialized, so search_function / search_class / search_class_method could not be used at all.

Changes

1. Clarify the path parameter description — it now explicitly states a directory (the codebase root) is required, and documents the file-path fallback. This is the root cause: the ambiguous description misled the model.
2. Defensive fallback in execute() — if a file path is passed and the file exists, resolve to its parent directory so the CKG index can still be built. Other non-directory paths (e.g. broken symlinks) still return the original clear error.

Why this is correct

• CKGDatabase.__init__ indexes via Path.glob("**/*"), so it needs a directory; passing the parent directory of the intended file is exactly what it expects.
• The fallback uses standard pathlib.Path.is_file() / .parent semantics and only triggers when the path exists but is not a directory, so existing directory behavior is unchanged.
• The _ckg_databases cache keys on the resolved directory Path, keeping caching consistent.

Fixes #354

[nankingjing]

Reviewed — the fallback to parent directory when a file path is given is a clean defensive fix for the CKG tool. Ready for review.

[nankingjing]

@chao-peng ready for review. Fixes CKG tool accepting a file path: falls back to parent directory when path is a file, and clarifies the path parameter description.