feat(deploy): give the PR reviewer its own image, like every other agent

pr-reviewer-1 was the one agent with no dedicated image and no compose
builder service — it reused roboco-agent-base, which left it absent from the
compose files entirely (so it looked like the PR reviewer simply was not
there). Make it first-class for parity: add docker/agent-pr-reviewer.Dockerfile
(FROM the base — read-only reviewer, no extra toolchain), an
agent-pr-reviewer-image builder service in both compose files and the registry
compose, the image in the release workflow's publish list, and map
pr-reviewer-1 -> roboco-agent-pr-reviewer in the orchestrator plus its
lazy-build dockerfile map. Supersedes the earlier base-reuse mapping.

.github/workflows/release.yml

@@ -99,6 +99,7 @@ jobs:
             [roboco-agent-doc]=docker/agent-doc.Dockerfile
             [roboco-agent-prompter]=docker/agent-prompter.Dockerfile
             [roboco-agent-secretary]=docker/agent-secretary.Dockerfile
+            [roboco-agent-pr-reviewer]=docker/agent-pr-reviewer.Dockerfile
           )
           for name in "${!IMAGES[@]}"; do
             echo "::group::build ${name}"