Post BeforeWire readiness summary to PRs

[senseb]

Adds a receipt-backed PR comment broker. The workflow now posts a readiness summary comment on same-repository PRs using an approval receipt and stable idempotency key, then uploads both broker evidence and receipt artifacts.

[senseb]

Reopening to retrigger the BeforeWire Agent Gate after broker permission-skip handling.

[senseb]

BeforeWire Agent Readiness

Status: pass

• Packets: 12/12 passed, 0 failed
• Receipt hash: sha256:6d9fcfc4df81001cd282f19d7861f74d2cfa257fdab96e234064ade71f1d0e6c
• Repository: beforewire/forkcell
• PR: #2
• CI event: pull_request
• CI SHA: e240244da916166b178c9c58d35dfb456958e68a
• Run: https://github.com/beforewire/forkcell/actions/runs/27906582336

Controls:

• agt_govern: pass
• github_shadow_gate: pass
• openshell_policy_validation: pass

Broker proof:

• Approval receipt: receipts/pr-comment-approval-receipt.json
• Idempotency key: 1047681c7daf79d113f171a4
• Broker target: github.issue_comment