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/issue-tracker-github.md

@@ -0,0 +1,34 @@
+# Issue tracker: GitHub
+
+Issues and PRDs for this repo live as GitHub issues. Use the `gh` CLI for all operations.
+
+## Conventions
+
+- **Create an issue**: `gh issue create --title "..." --body "..."`. Use a heredoc for multi-line bodies.
+- **Read an issue**: `gh issue view <number> --comments`, filtering comments by `jq` and also fetching labels.
+- **List issues**: `gh issue list --state open --json number,title,body,labels,comments --jq '[.[] | {number, title, body, labels: [.labels[].name], comments: [.comments[].body]}]'` with appropriate `--label` and `--state` filters.
+- **Comment on an issue**: `gh issue comment <number> --body "..."`
+- **Apply / remove labels**: `gh issue edit <number> --add-label "..."` / `--remove-label "..."`
+- **Close**: `gh issue close <number> --comment "..."`
+
+Infer the repo from `git remote -v` — `gh` does this automatically when run inside a clone.
+
+## Pull requests as a triage surface
+
+**PRs as a request surface: no.** _(Set to `yes` if this repo treats external PRs as feature requests; `/triage` reads this flag.)_
+
+When set to `yes`, PRs run through the same labels and states as issues, using the `gh pr` equivalents:
+
+- **Read a PR**: `gh pr view <number> --comments` and `gh pr diff <number>` for the diff.
+- **List external PRs for triage**: `gh pr list --state open --json number,title,body,labels,author,authorAssociation,comments` then keep only `authorAssociation` of `CONTRIBUTOR`, `FIRST_TIME_CONTRIBUTOR`, or `NONE` (drop `OWNER`/`MEMBER`/`COLLABORATOR`).
+- **Comment / label / close**: `gh pr comment`, `gh pr edit --add-label`/`--remove-label`, `gh pr close`.
+
+GitHub shares one number space across issues and PRs, so a bare `#42` may be either — resolve with `gh pr view 42` and fall back to `gh issue view 42`.
+
+## When a skill says "publish to the issue tracker"
+
+Create a GitHub issue.
+
+## When a skill says "fetch the relevant ticket"
+
+Run `gh issue view <number> --comments`.