Work diff canvas breaks when the user's Git config sets diff.mnemonicPrefix, color.diff=always or diff.noprefix #279

Expected and actual behavior / 实际现象与期望

The Work attempt diff (collect_git_delta in server/work_artifact_registry.py, and collect_diff in server/local_git.py for provider.diff without a ledger delta) should produce a standard a/ / b/ uncolored patch that parse_unified_diff can render, no matter how the user has configured git diff for their own terminal.

The patch commands only pass --no-ext-diff (and collect_diff not even that), so the user's diff configuration leaks into the machine-read patch:

• diff.mnemonicPrefix=true (a commonly recommended setting) makes git diff HEAD emit c/ / w/ prefixes. The canvas then shows the file as w/src/app.py, and the real path from changed_files is appended as a second, hunk-less "modified" entry.
• color.diff=always wraps every line in ANSI escapes (\x1b[1mdiff --git ...). No line matches a header or hunk, so the canvas shows only hunk-less entries from changed_files with 0 additions and 0 deletions, and the raw patch <pre> in the Work page shows escape codes.
• diff.noprefix=true drops the prefixes, so diff --git lines no longer have the shape the parser expects.
• diff.external is honored by collect_diff, so the patch returned by provider.diff can be any external tool's output.

Steps to reproduce / 复现步骤

In any Git workspace used for a Work attempt:

git config diff.mnemonicPrefix true      # or: git config color.diff always

Run an attempt that edits a tracked file (for example app.py), then open the attempt diff.

Actual (mnemonicPrefix): two entries, w/app.py with the hunks and app.py with none.
Actual (color.diff=always): one hunk-less entry app.py, additions == 0, deletions == 0.
Expected: one entry app.py with its hunks and counts.

The same can be seen directly:

git -c diff.mnemonicPrefix=true diff --no-ext-diff --unified=3 HEAD --   # diff --git c/app.py w/app.py

Relevant environment / 使用环境

Platform-independent; current main (fdd672e). Reproduced with Git 2.43 and 2.54. Affects any user whose global or repository Git config sets one of the options above.

Sanitized evidence / 错误信息或截图（请先脱敏）

git diff accepts --no-color --src-prefix=a/ --dst-prefix=b/ (and --no-ext-diff), which override these settings for a single command, so the Host can pin the format of the patches it parses without changing the user's configuration.

Related issues or fixes / 相关问题或修复

#274 / #275 fixed a different parser defect in the same diff canvas.