Add automated PR review via reusable workflow and orchestrator agent (#3)

* Add DDD agent grounded in Eric Evans' blue book

Reviews architecture for bounded context violations, ubiquitous language
drift, aggregate boundaries, domain events, misplaced logic, value objects,
repository boundaries, module structure, context relationships, and supple
design. Tactical heuristics folded under Evans' authority without co-branding.

* Add automated PR review via reusable workflow and orchestrator agent

Introduces a pr-review orchestrator agent that delegates to sub-agents
(security, design, testing, etc.) via the Task tool and aggregates
findings into a single PR comment. Includes a reusable GitHub Actions
workflow that any repo can call with a one-liner, authenticated via
GitHub Copilot models through GITHUB_TOKEN.

* Slim workflow prompt, widen failure condition, tighten docs

- Workflow prompt now passes only data (PR number + agent list);
  orchestration logic lives in the pr-review agent, not the YAML.
- Failure comment triggers on any step failure, not just the review step.
- Agent description no longer lists agents inline (points to agents/ dir).
- No-clobber install: project-level agents take precedence over shared ones.
- README: reframed motivation, added Operational Notes, linked to example file.
- Example workflow: added concurrency and cost notes in header.

* Pin opencode installer to v1.2.15