feat: add initial retro skill and OpenAI agent configuration

Author: mattpocock

skills/in-progress/retro/SKILL.md

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+---
+name: retro
+description: "STUB — not functional. Raw design ideas for a retrospective skill, awaiting a /writing-for-agents pass."
+disable-model-invocation: true
+---
+
+The user has asked for a **retrospective**. You are suggesting improvements to the coding agent's **environment** to improve future runs.
+
+## Steps
+
+1. Call the Skill tool with `writing-for-agents` for the writing style guide.
+
+2. Read the primary sources for the session the user specifies. This may mean searching through session logs on this machine. If the user doesn't specify a session, default to the current one.
+
+3. Look for candidates for improvement in these categories.
+
+- **Navigation**: how easy was it for the agent to find the right files? Are there hidden dependencies between files? Would a **navigation pointer** make it easier? _Use when_ the session took a long time to find a piece of information.
+- **Automated checks**: are there automated checks that could catch errors the agent made? Linting, typing, tests, filesystem linters? _Use when_ the agent made a mistake that could have been caught by an automated check.
+- **Coding standards**: should the **reviewer agent** be given a new rule to enforce? Should an existing rule be removed or clarified? _Use when_ the reviewer agent failed to catch a mistake.
+- **Global AGENTS.md**: are there any steering instructions that should be moved to coding standards (or automated checks) instead? _Use when_ the AGENTS.md file is particularly large - in the repo OR the user's global scope.
+- **Tool economy**: did the agent make expensive tool calls that could be streamlined? Is there any custom tooling (CLI's, MCP's) that is particularly token-inefficient? _Use when_ the agent made an expensive tool call.
+- **No-ops**: look for instructions in steering files that don't modify the agent's behavior. _Use when_ the steering files are large and unwieldy.
+
+4. Present these candidates to the user, in order of severity.
+
+## Reference
+
+### Implementation vs Review
+
+Remember that all work goes through two stages: implementation and review. The implementation agent has the most **context pressure**. They are responsible for exploration, writing code, and debugging failures.
+
+The review agent has the least context pressure - it receives a diff, so no exploration needed. It often does not need to write code or debug.
+
+This means that the review agent should be responsible for imposing coding standards, not the implementation agent.
+
+### Files
+
+You have access to several files in the repo:
+
+- `CLAUDE.md`/`AGENTS.md`: these files are pushed to the context window of any agent working in this repo. They should be used incredibly sparingly, usually only for **navigation pointers** to other files.
+- `CODING_STANDARDS.md`: this file is read during review, not implementation. Add **navigation pointers** to docs folders if the standards file gets more than 1,000 lines long.
+- Docs: use docs as references files, pointed to by other files. Look for existing docs before writing new ones.
+- Skills: use skills for docs (since their description goes into the agent's context window), or for user-invoked commands. Follow the advice in the `writing-for-agents` skill.

skills/in-progress/retro/agents/openai.yaml

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+interface:
+  display_name: "Retro (stub)"
+  short_description: "Not functional yet — design ideas only"
+policy:
+  allow_implicit_invocation: false