Add writing-for-agents skill documentation and related files

- Created SKILL.md and SKILL-MECHANICS.md for writing agent skills.
- Added openai.yaml for agent interface configuration.
- Introduced AGENTS.md for agent skills overview.
- Established CONTEXT.md for personal leave planning terminology.
- Documented ordered heuristic optimization in ADRs.
- Defined personal client-side planner limitations in ADRs.
- Created domain documentation for engineering skills.
- Added issue tracker guidelines for GitLab.
- Mapped triage labels for issue tracking.
- Initialized skills-lock.json to manage skill dependencies.

AGENTS.md

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+## Agent skills
+
+### Issue tracker
+
+Issues and specs live in GitLab Issues; use `glab` for tracker operations. See `docs/agents/issue-tracker.md`.
+
+### Triage labels
+
+Use the default canonical labels: `needs-triage`, `needs-info`, `ready-for-agent`, `ready-for-human`, and `wontfix`. See `docs/agents/triage-labels.md`.
+
+### Domain docs
+
+This is a single-context repo using root `CONTEXT.md` and `docs/adr/`. See `docs/agents/domain.md`.