none of the `check*` methods are compatible with gofumpt

`check_diff` should work once #640 is merged

## Summary
- Go tools (like `gofmt -d`) output diffs with `.orig` suffix on `---`
lines (e.g., `--- file.go.orig` instead of `--- file.go`)
- Adds `strip_orig_suffix()` to handle this by stripping `.orig` from
`---` lines when the corresponding `+++` line doesn't have it
- Correctly preserves `.orig` when the file itself has a `.orig`
extension

Supersedes #640 (rebased onto current main).

## Test plan
- [x] New bats test: `check_diff handles diffs with .orig suffix on ---
line`
- [x] New bats test: `check_diff works if the file has .orig suffix`
- [x] All existing `apply_check_diff.bats` tests pass

Co-authored-by: Joshua Cannon <josh.cannon@airbnb.com>

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Scoped change to diff parsing/apply that normalizes `--- *.orig`
headers; main risk is incorrectly rewriting paths in edge-case diffs,
mitigated by added tests.
> 
> **Overview**
> Adds support for Go-style unified diffs (e.g., `gofmt -d`) where only
the `---` header uses a `.orig` suffix by introducing
`strip_orig_suffix()` and running it before both file extraction
(`filter_files_from_check_diff`) and `git apply` (`apply_diff_output`).
> 
> Extends `apply_check_diff.bats` with coverage for `.orig`-suffixed
`---` lines and ensures files that actually end with `.orig` continue to
work, plus small test cleanup/renames.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
f86f004. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Joshua Cannon <josh.cannon@airbnb.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>