fix(mcp): support symbolic links with mode 120000 and target path content

[ManoharPaturi]

Closes #1768

Summary

Previously, getFileMode used stat() rather than lstat(). Because stat() follows symbolic links, fileStat.isSymbolicLink() was unreachable dead code, and commit_files would silently dereference symbolic links into regular file blobs. Furthermore, if getFileMode had used lstat() in isolation, Git tree entries would receive mode 120000 while readFile() would still supply the dereferenced file content instead of the link target path, corrupting the tree.

Changes

• Extracted getFileMode to src/mcp/file-mode.ts and implemented proper symlink detection via lstat() returning 120000.
• In commit_files (src/mcp/github-file-ops-server.ts), when a file has mode 120000, read the target path with readlink() and set the tree entry's content directly to the symlink target path rather than reading the dereferenced file bytes.
• Added comprehensive unit tests in test/file-mode.test.ts verifying mode resolution for regular files (100644), executables (100755), directories (040000), symbolic links (120000), and missing files fallback.

Verification

• bun test test/file-mode.test.ts passed (5/5).
• Full test suite bun test passed (971 pass, 0 fail).
• bun run typecheck passed cleanly.
• bun run format:check passed.

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