Migrate PR review to packaged cagent-action sub-actions

[derekmisler]

Summary

Migrates the PR review workflow from inline cagent YAML definitions to the new packaged docker/cagent-action sub-actions, dramatically reducing workflow complexity and maintenance burden.

Changes

• Delete inline agent configs: Removed .github/workflows/agents/pr-review.yaml and pr-review-feedback.yaml (221 lines of agent configuration)
• Simplify main review job: Replaced ~100 lines of workflow steps with single docker/cagent-action/review-pr@latest action call
• Simplify feedback learning job: Replaced ~80 lines with single docker/cagent-action/review-pr/learn@latest action call
• Remove manual orchestration: Eliminated context building, memory caching, reaction handling, and metadata gathering (now handled by the action)

Before vs After

Aspect	Before	After
Workflow lines	~200	~35
Agent config files	2 files (221 lines)	0 (packaged in action)
Memory management	Manual cache/restore	Handled by action
PR context building	Manual bash scripts	Handled by action

Test Plan (after merging to main, because that's how GHA works)

• Trigger a review by commenting /review on a PR
• Reply to an agent review comment to verify feedback learning works
• Verify review comments still contain <!-- cagent-review --> marker
• Check that APPROVE/REQUEST_CHANGES/COMMENT events are posted correctly