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/adr-wiki.md

@@ -0,0 +1,68 @@
+# ADR Wiki
+
+Architecture Decision Records live on the forge wiki and are cloned into `docs/adr/` during setup.
+
+## Wiki URL
+
+```
+<wiki-url>
+```
+
+Derived from the forge remote during `/setup-matt-pocock-skills`.
+
+## Bootstrap
+
+On first setup, `/setup-matt-pocock-skills` clones the wiki:
+
+```bash
+git clone <wiki-url> docs/adr/
+```
+
+And adds `docs/adr/` to `.gitignore`.
+
+On subsequent sessions, pull the latest:
+
+```bash
+cd docs/adr/ && git pull --rebase
+```
+
+## Auth
+
+Authentication for pushing ADR changes uses: `<auth-method>`
+
+- **Forge CLI token** — `gh` or `tea` already stores credentials; use them for git push via HTTPS with token auth.
+- **SSH key** — wiki remote uses `git@<host>:<owner>/<repo>.wiki.git`.
+- **Dedicated token** — stored in env var `GIT_WIKI_TOKEN`; used as password in HTTPS URL.
+
+## Session-end push
+
+At end of every session, the agent:
+
+1. `cd docs/adr/ && git add -A && git commit -m "docs(adr): <action> ADR-NNNN — <description>"`
+2. `git pull --rebase` (handle any web UI edits)
+3. `git push`
+
+If a conflict arises during rebase, surface it to the user for resolution.
+
+## Commit message convention
+
+```
+docs(adr): add ADR-NNNN — title
+docs(adr): update ADR-NNNN — reason
+docs(adr): remove ADR-NNNN — superseded by ADR-NNNN
+```
+
+## Consumer skills
+
+These skills read from `docs/adr/` by relative path — the wiki clone is transparent:
+
+- `diagnosing-bugs`
+- `tdd`
+- `improve-codebase-architecture`
+- `domain-modeling`
+- `grill-with-docs`
+
+These skills may create ADRs in `docs/adr/`; the wiki push is handled at session end:
+
+- `domain-modeling`
+- `improve-codebase-architecture`