Task: add repository agent instructions

AGENTS.md

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+# Agent Collaboration Policy
+
+Use subagents for implementation work that introduces multiple independent
+features or components. Keep basic, single-purpose tasks in the main agent
+unless delegation clearly improves the outcome.
+
+## Model selection
+
+- **Basic tasks:** delegate to a fast, economical model such as `gpt-5.6-luna`.
+- **Complex implementation tasks:** delegate to a balanced model such as
+  `gpt-5.6-terra`.
+- **Code reviews and very complex tasks:** delegate to a frontier model such
+  as `gpt-5.6-sol`.
+
+When the available model names differ, choose the closest equivalent by the
+same capability tier: fast/economical, balanced/complex, then frontier/review.
+
+## Delegation practice
+
+- Split only work that is independently actionable, with clear file or module
+  ownership, to avoid overlapping edits.
+- Retain final integration, verification, and user communication in the primary
+  agent.
+- Do not use subagents for simple edits, one-off questions, or narrowly scoped
+  diagnostics unless the user explicitly requests delegation.
+
+## Feature delivery and pull requests
+
+- Deliver new features through pull requests.
+- Once a requested task is complete and verified, automatically commit the
+  relevant changes, push the branch, and open a pull request unless the user
+  explicitly asks not to.
+- Do not create a pull request for a question, investigation, review-only task,
+  or an intentionally uncommitted work-in-progress.
+
+## Pull request descriptions
+
+Write a detailed but straightforward PR body for an AI engineer who does not
+need to read the implementation to understand the change. Use plain language
+and explain:
+
+- what changed from a user's perspective;
+- why it matters;
+- how to verify it, including any important limitations or follow-up work.
+
+Avoid implementation jargon, internal file names, framework details, and code
+walkthroughs unless they are essential to using or reviewing the feature. Keep
+the description focused: include enough context to make the decision clear,
+without turning it into a technical design document.