Provide a role-scoped GitHub gateway for Docker automations

Desired Behavior

Provide a repository-scoped GitHub gateway for Docker automations. The trusted control plane retains the GitHub credential, while triage, implementation, review, and watchdog workers receive separate role tokens. This is the independently reviewable permission boundary used by the software-factory recipe in #556.

Acceptance Criteria

• Role allowlists restrict writes to issue readiness, factory implementation branches, exact-commit review evidence, or guarded merge according to the selected role.
• Private repository snapshots are available for exact commits without handing the GitHub credential to a worker.
• Merge rejects changed heads, stale bases, missing acceptance statuses, failing/pending checks, and conflicts, and includes the reviewed SHA in the merge request.
• Workers cannot administer/delete repositories, write main directly, force push, or invoke an unguarded merge.
• Authorization and fail-closed merge tests pass without credentials or network access; setup documents the trust boundary and limits.

ready-for-dev: scope and acceptance criteria are defined.

Start by reading the software-factory recipe in #556 and tracing how its triage, implementation, review, and watchdog workers currently access GitHub. Define the gateway against every listed acceptance criterion, then verify authorization and fail-closed merge behavior with tests that use neither credentials nor network access.