/regression-guard — emit a REGRESSION-GUARD test stub for a refactor commit

Thin wrapper around the regression-guard binary (installed at ~/.local/bin/regression-guard; built by install.sh). Binary owns: git show of the commit, diff pattern auto-detection, test skeleton rendering. Agent fills in the test body.

Steps

1. Invoke the binary with the refactor commit hash:

   regression-guard <commit-hash> \
                    [--pattern <override>] \
                    [--test-name <fn-name>] \
                    [--module-path <crate::path::to::mod>] \
                    [--git-dir <repo-path>]

   Stdout is a Rust source snippet (test fn + REGRESSION-GUARD docstring referencing the commit + the detected pattern).

2. Paste the snippet into the test module of the file the refactor touched (typically src/<file>.rs's #[cfg(test)] mod tests block).
3. Fill in the test body with the actual exercise of the pathological input the refactor defends against. Examples per pattern are listed in the snippet's TODO comment.
4. Run the test: cargo test <test_name> — it MUST pass on the post-refactor code (that's the point — it locks in the new behaviour). If it fails, the test isn't asserting the right thing; revisit.
5. Commit the test in the same PR / commit-chain as the refactor, or in a [REGRESSION-GUARD]-prefixed follow-up commit referencing the refactor hash.

Net visible tool calls per regression-guard

2–4 total: Bash (regression-guard) + Edit (paste snippet) + Bash (cargo test) + optional Bash (commit).

Down from ~6 in the prior manual flow (read commit, identify pattern in head, write test from scratch, write docstring, copy hash, etc.).

Detected patterns

Auto-detection covers the common cases from the LFI 2026-05-19 doctrine sweep (31 violations cleared across 9 files). For patterns not covered, pass --pattern unknown and edit the docstring manually.