v0.3.0

Highlights\n\n- Adds local-key signed evidence bundle sidecars with evidence sign and evidence verify-signature.\n- Keeps unsigned bundles fully supported.\n- Documents the signed-bundle boundary: local tamper detection for exact bundle bytes only; no command-execution, artifact-semantics, signer-identity, keyserver, or trust-chain claim.\n- Adds signed-sidecar tutorial coverage and regression tests.\n\n## Validation\n\n- PR #21 CI passed.\n- Local validation on merged main: uv run pytest -q and uv run --extra schema pytest -q.\n