Add a setup-commands input to the cargo mutation workflow

[leynos]

Summary

Callers cannot inject steps into a workflow_call job, so repositories whose builds need job-level preparation (linkers, system packages) had no way to satisfy it before cargo-mutants runs. The concrete case is review feedback on agent-template-rust#52: generated repos render .cargo/config.toml with -fuse-ld=mold and their CI installs mold before any cargo command — the mutation baseline would fail to link on a bare runner.

This branch adds a setup-commands input (default empty) to .github/workflows/mutation-cargo.yml, executed in each mutants job after toolchain setup via a temp script under set -euo pipefail, and documents it in the caller guide. The commands run with the same privileges the caller's own workflows already hold.

Validation

• YAML parse, actionlint, zizmor: clean
• make markdownlint: 0 errors

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: e768976c-e768-43aa-832e-61e9a92748bb

📥 Commits

Reviewing files that changed from the base of the PR and between bf19d93 and 96e4fad.

📒 Files selected for processing (2)

• .github/workflows/mutation-cargo.yml
• docs/mutation-cargo-workflow.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch add-mutation-setup-commands

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