Extend GitHub PR reviewer with independent acceptance checks

[neubig]

Desired Behavior

Extend the existing github-pr-reviewer automation with a lightweight scanner that delegates each labeled PR head to an agent. The scanner must not create, attach, or poll conversations. The Automation service owns conversation routing and runtime lifecycle; the profile-backed agent reuses the established review workflow inside its sandbox and publishes a readable native review plus exact-head review and test decisions.

Acceptance Criteria

• The existing reviewer prompt and native GitHub review workflow are reused.
• Each labeled exact PR head is submitted under a stable repository-and-PR identity and idempotency key.
• The agent reviews the exact head, runs appropriate tests, publishes readable findings, and records exact-head software-factory/review and software-factory/tests statuses.
• The reviewer token needs no repository contents write permission.
• The scanner contains no Agent Server URL, session key, conversation attachment, runtime polling, checkout, or workspace-kind branching.
• Focused tests and live Agent Canvas evidence cover approval and changes-requested outcomes.

[github-actions]

✅ ready-for-dev label applied

This issue meets the readiness criteria and is ready for contribution.
A contributor can now open a PR that links this issue (Fixes #<number>).

This is an automated readiness check posted by a workflow.