fix(engine): verify SLSA build provenance via sigstore-verify (ADR-0020)

[thejefflarson]

What was wrong

The dashboard's build-provenance column was blank on every row — the observer resolved Absent for every image, including protector's own signed+attested images. The provenance feature (ADR-0020 §5) had been inert since it shipped.

Root cause — two independent dead ends in the sigstore crate (0.14, newest release)

1. trusted_signature_layers fetches the SLSA bundle as an OCI referrer, then from_sigstore_bundle rejects it — it hardcodes the cosign sign/v1 predicate and errors on https://slsa.dev/provenance/v1. So the shared signing fetch can never surface a SLSA layer.
2. The crate's other public path, bundle::verify::Verifier, recomputes the Rekor DSSE envelopeHash from a proto round-trip that never equals the hash Rekor stored (a canonicalization bug) — offline DSSE always fails; the online path is an unimplemented stub.

Both confirmed against the live agent image (envelopeHash mismatch). No published sigstore version fixes either, and the Fulcio-chain / SCT primitives needed to hand-roll around them are pub(crate).

Fix

Add sigstore-verify (prefix-dev) for the provenance verify step only — a maintained verifier purpose-built for GitHub artifact attestation that handles the DSSE Rekor-entry consistency correctly and ships a built-in production trust root. Signing stays on the sigstore crate, untouched.

New provenance_observer module: fetch the image's OCI referrers directly, select the slsa.dev/provenance bundle, verify it offline against the same registry (no new egress destination, no transparency-log call), and feed the verified facts into the unchanged classify_provenance / parse_slsa_predicate pipeline — so baseline, drift, and render are all untouched; only the source of the facts moved.

• Registry without the referrers API → Absent, not Checking (the manifest fetch already proved reachability), so mirrored base images stay calm.
• No new PROTECTOR_* flag — detection stays on by default.

Validated live

• protector-agent:0.14.1 → Verified(source: github.com/thejefflarson/protector, builder: …/agent.yml@refs/tags/v0.14.1)
• redis (mirror), ollama, argocd → Absent

Tests & gates

Full engine suite (1233) green · clippy clean (warnings-as-errors) · fmt clean · self-containment guard passes · files under the 1000-line cap. Adds 7 unit tests + 2 #[ignore]d live integration tests guarding the exact green-on-fixtures / dead-in-prod failure mode. ADR-0020 amended with the decision (and its two now-stale notes corrected).

ADR-0042 note: provenance_facts / predicate_from_pae were removed from cosign.rs, but classify_provenance / parse_slsa_predicate gained provenance_observer as their live consumer — no shared layer dropped to zero.

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