feat(skills): add engineering skill definitions under common/engineering/ all from @mattpocock

Add 10 skill definitions covering codebase design, domain modeling,
test-driven development, triage, and development workflows:

- codebase-design: vocabulary and patterns for designing deep modules
- domain-modeling: build and sharpen project domain models
- grill-with-docs: interview-based design refinement with documentation
- improve-codebase-architecture: scan and report on deepening opportunities
- resolving-merge-conflicts: structured approach to merge/rebase conflicts
- setup-skills: per-repo configuration for engineering skills
- tdd: test-driven development workflow with vertical slices
- to-issues: break plans into tracer-bullet vertical slice issues
- to-prd: synthesize conversations into PRDs
- triage: state machine for issue/PR triage roles

Includes supporting files for ADR formats, issue tracker integrations
(GitHub, GitLab, Gitea), triage labels, and agent brief templates.

common/engineering/tdd/refactoring.md

@@ -0,0 +1,10 @@
+# Refactor Candidates
+
+After TDD cycle, look for:
+
+- **Duplication** → Extract function/class
+- **Long methods** → Break into private helpers (keep tests on public interface)
+- **Shallow modules** → Combine or deepen
+- **Feature envy** → Move logic to where data lives
+- **Primitive obsession** → Introduce value objects
+- **Existing code** the new code reveals as problematic