Guardrails: YAML lint, build-pre-test, pre-commit .perf detection

[alderpath]

Three guardrails to prevent CI failure types found in #4:

1. YAML workflow lint — validates all .github/workflows/*.yml with python3 yaml.safe_load before test run. Catches colon-in-shell issues.
2. Build check before test — cargo check --release runs before cargo test. Catches compile errors 5 min faster.
3. Pre-commit stale .perf detection — blocks commits that include .perf files.
4. Pre-commit YAML lint — same check client-side before push.

[alderpath]

Closing — superseded by later commits that added YAML linting via CI step and pre-commit hook separately. This branch would remove the existing stale-file detector and gitleaks check.