file_permission_handler previews diverge from actual file_modifications behavior - approved diff is not the applied diff (replace-all vs replace-first; overwrite preview never reads old content)

File: code_puppy/plugins/file_permission_handler/register_callbacks.py (_preview_delete_snippet, _preview_write_to_file, _preview_replace_in_file, ~lines 72-180) vs code_puppy/tools/file_modifications.py (_delete_snippet_from_file, _write_to_file, _replace_in_file)

Severity: Medium (correctness - the diff the user approves is not the diff that gets applied)

The permission plugin re-implements the modification logic to build preview diffs, and the copies have diverged from the real implementations:

1. _preview_delete_snippet uses original.replace(snippet, "") - replaces all occurrences. The actual _delete_snippet_from_file uses original.replace(snippet, "", 1) - first occurrence only. If the snippet appears more than once, the user approves a diff showing N deletions but only 1 is applied.

2. _preview_replace_in_file uses modified.replace(old_snippet, new_snippet) (all occurrences); the real _replace_in_file uses modified.replace(old_snippet, new_snippet, 1) (first only). Same approve-X-get-Y mismatch. The fuzzy-window fallback is also duplicated (~40 lines) with its own subtle ordering difference (had_trailing_newline captured at a different point).

3. _preview_write_to_file diffs against [""] when the file exists:

diff_lines = difflib.unified_diff(
    [] if not exists else [""],   # <-- never reads the real file!
    content.splitlines(keepends=True), ...)

The real _write_to_file reads the existing file content. So for any overwrite, the preview shows the entire new file as additions with no deletions - the user never sees what they are about to destroy, which defeats the purpose of the approval prompt.

Suggested fix (DRY): extract pure "compute (modified_content, diff)" functions in file_modifications.py (no write), have both the real apply path and the permission preview call them. Then the preview is the applied diff by construction.

Filed by Zen Reviewer B (code-puppy-60635a)

Start by comparing _preview_delete_snippet, _preview_write_to_file, and _preview_replace_in_file in code_puppy/plugins/file_permission_handler/register_callbacks.py with their counterparts in code_puppy/tools/file_modifications.py. Trace how each preview and apply path computes content and diffs, then verify that repeated snippets use first-only behavior and overwrites include the existing file content in the approved diff.