Add AGENTS guidelines for coding agents.

Author: mikecao

AGENTS.md

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+# AGENTS
+
+This repository supports automated and human-assisted coding agents.
+
+## Scope
+
+These guidelines apply to the whole repository unless a more specific `AGENTS.md` exists in a subdirectory.
+
+## Core Rules
+
+1. Keep changes focused and minimal.
+2. Follow existing code style and project conventions.
+3. Do not introduce unrelated refactors in the same change.
+4. Update docs when behavior, APIs, or workflows change.
+5. Prefer safe, reversible changes.
+
+## Workflow
+
+1. Read relevant files before editing.
+2. Make the smallest change that solves the task.
+3. Run targeted checks for touched areas.
+4. Summarize what changed and why.
+
+## Validation
+
+- Run tests/lint only as needed for the affected scope.
+- If full validation is expensive, run focused checks and call out what was not run.
+
+## Commit and PR Guidance
+
+- Use clear commit messages that explain intent.
+- Keep pull requests small and reviewable.
+- Include migration notes if configs or data shape changed.
+
+## Security and Safety
+
+- Never commit secrets or credentials.
+- Avoid destructive commands unless explicitly requested.
+- Prefer least-privilege defaults for new configs and scripts.