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/setup-skills/triage-labels.md

@@ -0,0 +1,15 @@
+# Triage Labels
+
+The skills speak in terms of five canonical triage roles. This file maps those roles to the actual label strings used in this repo's issue tracker.
+
+| Label in skills | Label in our tracker | Meaning                                  |
+| -------------------------- | -------------------- | ---------------------------------------- |
+| `needs-triage`             | `needs-triage`       | Maintainer needs to evaluate this issue  |
+| `needs-info`               | `needs-info`         | Waiting on reporter for more information |
+| `ready-for-agent`          | `ready-for-agent`    | Fully specified, ready for an AFK agent  |
+| `ready-for-human`          | `ready-for-human`    | Requires human implementation            |
+| `wontfix`                  | `wontfix`            | Will not be actioned                     |
+
+When a skill mentions a role (e.g. "apply the AFK-ready triage label"), use the corresponding label string from this table.
+
+Edit the right-hand column to match whatever vocabulary you actually use.