fix(targethasher): frame directory hashes for BUG-007

[sbalabanov]

Summary

BUG-007 fixes directory hashes for edge cases that file contents alone miss:

• A file, directory, or symlink rename changes the hash.
• Directory nesting and empty directories change the hash.
• Adding a symlink or changing its target changes the hash.
• Creation order does not change the hash.

The implementation streams filepath.WalkDir in lexical order and hashes each entry's local name, kind, and file contents or symlink target. It does not hash checkout paths or filesystem mode bits, so the result stays stable across machines.

Tests

Added coverage for rename, directory structure, symlinks, creation order, and mode independence.

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[sbalabanov]

Update PR description to correctly represent changes on the last version vs base revision. Make accent on edge cases it aims to address. Shorten. Use simple language.

[sbalabanov]

Updated the BUG-007 commit message and PR description in fd67123 to match the current two-file diff. The shorter text now highlights directory renames and structure, symlink target changes, deterministic streaming traversal, and the exclusion of checkout paths and mode bits. No code changed.

[addressed by agent]