cleanup-build-artifacts

# Cleanup Build Artifacts Sweep up the transient artifacts *this task* produced so the working tree and disk stay clean. Implements the AGENTS.md / AGENTS.md rule: "Clean up build artifacts you produced once the task is done." ## Scope **In scope** — artifacts created during this task: - Release binaries built for one-off verification (`target/release/<bin>`) - `cargo mutants` output (`mutants.out/`, `mutants.out.old/`) - `cargo llvm-cov` reports (`target/llvm-cov-target/`, generated `lcov.info` / HTML report dirs) - Scratch files at the repo root: `pr.diff`, `*.diff`, ad-hoc `*.tar`/`*.tar.gz`, throwaway `*.log`, `scratch_*`, `tmp_*` - One-off binaries built outside `target/` (e.g. via `--out-dir`) - Temporary fixture dirs created for an investigation that aren't checked in **Out of scope — DO NOT TOUCH:** - The persistent `target/` debug tree being actively iterated on (`cargo build`, `cargo test` incremental output). Leave it. - Anything tracked by git that's modified (M) or staged — those are real work. - `.ironlint/` runtime state unless the task explicitly created it for a one-off check. - Pre-existing files in the working tree the task didn't author. If unsure whether something is yours, leave it. ## Instructions ### Step 1: Enumerate what this task produced Before cleaning, list candidates. Run these in parallel: ```bash # Untracked files that appeared during the task git status --short --untracked-files=normal # Release binaries (likely candidates if you ra...