feat: add portable PoCI external witness SDK

[safal207]

What changed

Adds the eleventh PoCI slice: a portable onboarding kit for an independently owned external witness repository.

The kit contains:

• one dependency-free Python SDK;
• the exact organizational-independence challenge fixture and challenge root;
• a strict external-operator response schema;
• a copyable GitHub Actions workflow for another owner or organization;
• 13 focused conformance, mutation, identity, determinism, and submission tests;
• documentation with a minimal onboarding path;
• a dedicated SDK workflow that verifies the live producer attestation, recomputes the six-graph space, proves the current same-owner run remains HOLD, builds a deterministic kit manifest, keyless-attests that manifest, and uploads a distributable artifact.

External operator flow

verify challenge root
        ↓
verify ProofPath producer attestation
        ↓
recompute source + six graph roots + three transition cells
        ↓
create external response subject
        ↓
keyless-attest exact response bytes
        ↓
verify the response attestation
        ↓
finalize portable submission package

Honest same-owner behavior

This repository cannot manufacture an independent operator response for itself. The live SDK conformance workflow is expected to produce:

decision: HOLD
reason: OPERATOR_OWNER_NOT_INDEPENDENT
claims_organizational_independence: false
authority_granted: false

It attests only the distributable SDK kit manifest, not a fake external response.

Trust boundary

The reference workflow runs in another repository and gets a separate owner/workflow/Sigstore identity. It currently reuses the pinned ProofPath graph builder as a reproducible reference implementation. It therefore demonstrates owner-diverse execution, not yet independently implemented algorithm diversity.

Stacked PR

Base: agent/poci-organizational-independence-gate-v0.1 / #202

The kit is intended for the external operator requested in issue #203.

No merge is performed by this PR.

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[safal207]

Validation complete

Final head: 469d742cfe59b12875c5c6e1fc0ce660c38ab1dd

Passed:

• PoCI External Witness SDK run Write ProofPath HTTP Profile v0.1 #1 (30628834750)
  • 13 focused conformance, mutation, identity, determinism, and submission tests;
  • exact challenge-root verification;
  • live six-graph recomputation;
  • pinned ProofPath producer-attestation verification;
  • same-owner conformance result remained HOLD with only OPERATOR_OWNER_NOT_INDEPENDENT;
  • deterministic five-file SDK kit manifest;
  • keyless attestation of the kit manifest;
  • distributable artifact upload.
• PoCI run Community experiment L5: Protocol and threat model critique #19 — success.
• Rust run #286 — formatting, clippy, workspace tests, Compute Witness, metrics, reusable action, Personal Agent Guard, and Agent Payment Guard all succeeded.

Distributable evidence artifact:

• name: poci-external-witness-sdk-30628834750
• artifact id: 8792486498
• size: 23037 bytes
• SHA-256: 0f7a33017c07a6d548f4f645d9caee7d600e657228882f531a514429a3ff01f5
• expires: 2026-08-14T11:57:26Z

Honest result: this repository attested the SDK kit, not an independent operator response. A same-owner run cannot claim organizational independence. No merge has been performed.