centralize guides and rails under mosaic with runtime compatibility links

templates/agent/projects/python-library/AGENTS.md.template

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+# ${PROJECT_NAME} — Agent Context
+
+> Patterns, gotchas, and guidelines for AI agents working on this project.
+> **Update this file** when you discover reusable patterns or non-obvious requirements.
+
+## Codebase Patterns
+
+- All public APIs must have type hints and docstrings
+- Zero or minimal runtime dependencies — be conservative adding deps
+- Exports defined in `__init__.py`
+<!-- Add project-specific patterns as you discover them -->
+
+## Common Gotchas
+
+- Always run `uv sync` after pulling — dependencies may have changed
+- Ensure backward compatibility — this is a library consumed by other projects
+<!-- Add project-specific gotchas -->
+
+## Quality Gates
+
+**All must pass before any commit:**
+
+```bash
+uv run ruff check src/ tests/ && uv run ruff format --check src/ && uv run mypy src/ && uv run pytest --cov
+```
+
+## Key Files
+
+| File | Purpose |
+|------|---------|
+| `pyproject.toml` | Project configuration, dependencies, build |
+| `src/${PROJECT_SLUG}/__init__.py` | Public API exports |
+<!-- Add project-specific key files -->
+
+## Testing Approaches
+
+- Unit tests: pytest with fixtures in `conftest.py`
+- Coverage minimum: 85%
+<!-- Document project-specific testing patterns -->