Acceptance tests for supply-chain flows

[michael-herwig]

Part of #24.

What

Add to test/tests/:

1. test_trust_policy.py — identity pin match / mismatch.
2. test_install_auto_verify.py — install respects [trust.policy].
3. test_sbom_attach_discover.py — push with --sbom, fetch via ocx sbom.
4. test_slsa_provenance.py — attach + verify SLSA referrer.
5. test_osv_scan.py — install of binary with cargo-auditable data triggers expected scan path.
6. test_mirror_signing.py — mirror bundle is signed end-to-end.
7. test_referrers_capability.py — clear error against non-referrers registry.
8. test_cosign_interop.py — cosign verify accepts signatures produced by ocx sign and vice versa (regression suite).

All tests reuse the fake Fulcio + Rekor + OIDC fixture from PR #87 (test/tests/fixtures/fake_sigstore.py).

Acceptance criteria

• All tests green in task test.
• Cosign-interop test runs against real cosign binary (installed via ocx install cosign:2.6+).
• Fixture extensions kept minimal — reuse over duplication.

[michael-herwig]

Closing — redistributed per the milestone split (plan row 18).

A monolithic "acceptance tests" issue contradicts contract-first TDD: tests are the specification and land with the feature they specify, not as a trailing batch. Each test becomes a named acceptance criterion of its owning issue:

Test	Now an acceptance criterion of
test_trust_policy.py	#98
test_install_auto_verify.py	#99
test_sbom_attach_discover.py	#100 + #101
test_slsa_provenance.py	#102 + #103
test_osv_scan.py	#104
test_mirror_signing.py	already migrated → ocx-sh/ocx-mirror#7
test_referrers_capability.py	#106
test_cosign_interop.py	#197 (standalone interop suite, targets cosign v3)

The cross-cutting note — reuse the fake Fulcio + Rekor + OIDC fixture (test/tests/fixtures/fake_sigstore.py), keep fixture extensions minimal — is copied into the #24 and #199 tracker bodies so it survives this issue.