ci(agent-policy): select central channel

[willgriffin]

Closes #6.

Summary

• Replace the repository artifact pin with the protected HappyVertical policy channel selector.
• Verify the selected signed artifact and manifest at runtime.

Validation

• scripts/hv-agent audit --repo .
• actionlint .github/workflows/agent-policy.yml
• git diff --check
• Independent lifecycle and code review cleared the final diff.

{"schema":"hv-agent-run:v1","runtime":"codex","owner":"willgriffin","session":"codex-policy-channel-gen13-will-github-6-r3","issue":6,"branch":"codex/issue-6-policy-channel-gen13","worktree":"/tmp/hv-policy-gen13.BOAD8e/willgriffin-.github","policy_profile":"personal","policy_revision":"1.0.0","validation":["hv-agent audit","actionlint","git diff --check","two reviewer approvals"]}