ci: add keyless multi-runner PoCI witness network

[safal207]

What changed

Adds the eighth PoCI slice as a deliberately small trust-layer PR:

• runs witness-alpha, witness-beta, and witness-gamma as three separate GitHub-hosted matrix jobs;
• emits one portable multi-graph witness statement per runner;
• creates a keyless SLSA provenance attestation for each statement with actions/attest@v4;
• downloads the three statements into a separate coordinator job;
• requires gh attestation verify to validate the exact repository, signer workflow, source digest, signer workflow digest, GitHub OIDC issuer, and hosted-runner boundary;
• supplies only the verified statement bytes to the exact-root quorum engine introduced in feat: add isolated PoCI multigraph witness quorum #199;
• requires all three canonical statements to match the same six graph roots and transition-cell root under the existing 2-of-3 policy;
• creates and attests a final attested-quorum-report.json;
• uploads the statements, full CLI verification results, and final report as one evidence artifact.

Why keyless Sigstore instead of committed test keys

No long-lived signing key is stored in the repository. GitHub OIDC mints the short-lived signing identity, Sigstore signs the artifact provenance, and the coordinator pins the signer workflow and source commits during verification.

Trust boundary

This demonstrates separate runners and cryptographically verified workflow provenance. It does not claim that the three jobs are operated by independent organizations: the same repository still controls all three jobs. A real network boundary requires at least one externally governed signer workflow or consumer.

Stacked PR

Base: agent/poci-multigraph-witness-quorum-v0.1 / #199

Review and merge after #199, then retarget to main if GitHub does not do so automatically.

Reviewer evidence

Expected final artifact layout:

statements/witness-alpha.json
statements/witness-beta.json
statements/witness-gamma.json
attestation-results/witness-alpha.json
attestation-results/witness-beta.json
attestation-results/witness-gamma.json
attested-quorum-report.json

[safal207]

Validation complete on a726f2a8b3bd9c6c4469be542e30bb54b230d510.

Passed:

• PoCI Signed Witness Network run Write ProofPath HTTP Profile v0.1 #1
• PoCI run Add real model agent demo #13
• Rust run #280

The signed-witness workflow completed all three witness jobs, coordinator verification, exact-root quorum, final report attestation, and evidence upload.

Final artifact: poci-signed-witness-network-30617244808 (id 8787874915, SHA-256 3874fb7c66f47a76813a04120bab096b198e57351046fd264ec13d6b0dd21b9f, expires 2026-08-14).