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-gitea.md

@@ -0,0 +1,35 @@
+# Issue tracker: Gitea
+
+Issues and PRDs for this repo live as Gitea issues. Use the `gitea` CLI for all operations.
+
+## Conventions
+
+- **Create an issue**: `tea issue create --title "..." --description "..."`. Use a heredoc for multi-line descriptions. 
+- **Read an issue**: `tea issue <number> --comments`. Use `-o json` for machine-readable output.
+- **List issues**: `tea issue list --state open -o json` with appropriate `--labels` and `--state` filters. 
+- **Comment on an issue**: `tea comment <number> "..."`.
+- **Apply / remove labels**: `tea issue edit <number> --add-label "..."` / `--remove-label "..."`. Multiple labels can be comma-separated or by repeating the flag.les
+- **Close**: `tea issue close <number>`. `tea issue close` does not accept a closing comment, so post the explanation first with `tea comment <number> "..."`, then close.
+
+Infer the repo from git remote -v — `tea` 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**: `tea pr <number> --comments` and `tea api /repos/{owner}/{repo}/pulls/<number>.diff`  for the diff.
+
+- **List external PRs for triage**: `tea pr list --state open -o json ` then keep only PRs whose author is not a project member/owner (a contributor's MR, not a maintainer's in-flight work).
+- **Comment / label / close**: `tea comment <number> "..."`, `tea pr edit --add-label`/`--remove-label`, `tea pr close`.
+
+Gitea shares one number space across issues and PRs, so a bare `#42` may be either — resolve with `tea pr 42` and fall back to `tea issue 42`.
+
+## When a skill says "publish to the issue tracker"
+
+Create a Gitea issue.
+
+## When a skill says "fetch the relevant ticket"
+
+Run `tea issue <number> --comments`.