fix: refuse rewriting through symbolic links

[hc-ui]

Directory walks still skip file symlinks (so a stray link cannot pull an outside path into a repo-wide scan/fix). Explicitly named file symlinks are now analyzed, but fix --apply and attributes --write refuse to write through a symlink (no replacing the link, no appending to the link target).

This is a different gap from #1 (default-explain / check unreadable / GitContext cwd) and #2 (PowerShell CRLF).

Changes

• collect_files no longer silently skips a named file symlink.
• apply_action and attributes --write error instead of following the link.
• Tests cover named-link analysis, refused fix --apply, and refused .gitattributes write.
• No version bump.

Tests

PYTHONPATH=. python3 -m pytest -q → 155 passed.

[hc-ui]

同一份修改已经基于当前 main（含 #1/#2）重新开出并合并：#4

这个 PR 和已合并的 #1/#2 冲突，所以改关掉，不是修改丢了。