fix(opencode): stabilize git diff patch prefixes

[YOMXXX]

Issue for this PR

Closes #29203

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Git's diff.mnemonicprefix=true changes patch headers from the usual a/ and b/ prefixes to contextual prefixes such as c/ and w/. Those patches can fail downstream diff rendering even though the file list is correct.

This PR forces the git patch commands used by OpenCode to emit stable a/ and b/ prefixes without changing the user's repository config. It covers single-file, batched, and untracked patch generation paths.

How did you verify your code works?

• bun test test/project/vcs.test.ts -t "ignores mnemonic prefix" from packages/opencode
• bun test test/project/vcs.test.ts from packages/opencode
• bun typecheck from packages/opencode
• bun turbo typecheck from the repo root
• .husky/pre-push

Screenshots / recordings

No UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR