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/skills/implement/SKILL.md

@@ -0,0 +1,15 @@
+---
+name: implement
+description: "Implement a piece of work based on a spec or set of tickets."
+disable-model-invocation: true
+---
+
+Implement the work described by the user in the spec or tickets.
+
+Use /tdd where possible, at pre-agreed seams.
+
+Run typechecking regularly, single test files regularly, and the full test suite once at the end.
+
+Once done, use /code-review to review the work.
+
+Commit your work to the current branch.