Python: Fix prek runner duplication and add skills (#3791)

* Python: fix prek runner running fmt/lint in all packages on core change

When a core package file changed, run_tasks_in_changed_packages.py ran
fmt, lint, and pyright in ALL 22 packages (66 tasks). Only type-checking
tasks (pyright, mypy) need to propagate to all packages since type
changes in core affect downstream packages. File-local tasks (fmt, lint)
only need to run in packages with actual file changes.

This reduces a core-only change from 66 tasks to 24 tasks (2 local +
22 pyright).

Also adds no-commit-to-branch builtin hook to protect the main branch
from direct commits.

* Python: add agent skills extracted from AGENTS.md and coding standards

Add 5 skills to python/.github/skills/ following the Agent Skills format:
- python-development: coding standards, type annotations, docstrings, logging
- python-testing: test structure, fixtures, running tests, async mode
- python-code-quality: linting, formatting, type checking, prek hooks, CI
- python-package-management: monorepo structure, lazy loading, versioning
- python-samples: sample structure, PEP 723, documentation guidelines

* Python: deduplicate AGENTS.md and instructions with agent skills

* updated skills

* fixes from review

* Python: increase timeout for web search integration test

python/.github/skills/python-testing/SKILL.md

@@ -0,0 +1,84 @@
+---
+name: python-testing
+description: >
+  Guidelines for writing and running tests in the Agent Framework Python
+  codebase. Use this when creating, modifying, or running tests.
+---
+
+# Python Testing
+
+We strive for at least 85% test coverage across the codebase, with a focus on core packages and critical paths. Tests should be fast, reliable, and maintainable.
+When adding new code, check that the relevant sections of the codebase are covered by tests, and add new tests as needed. When modifying existing code, update or add tests to cover the changes.
+We run tests in two stages, for a PR each commit is tested with `RUN_INTEGRATION_TESTS=false` (unit tests only), and the full suite with `RUN_INTEGRATION_TESTS=true` is run when merging.
+
+## Running Tests
+
+```bash
+# Run tests for all packages in parallel
+uv run poe test
+
+# Run tests for a specific package
+uv run --directory packages/core poe test
+
+# Run all tests in a single pytest invocation (faster, uses pytest-xdist)
+uv run poe all-tests
+
+# With coverage
+uv run poe all-tests-cov
+```
+
+## Test Configuration
+
+- **Async mode**: `asyncio_mode = "auto"` is enabled — do NOT use `@pytest.mark.asyncio`, but do mark tests with `async def` and use `await` for async calls
+- **Timeout**: Default 60 seconds per test
+- **Import mode**: `importlib` for cross-package isolation
+
+## Test Directory Structure
+
+Test directories must NOT contain `__init__.py` files.
+
+Non-core packages must place tests in a uniquely-named subdirectory:
+
+```
+packages/anthropic/
+├── tests/
+│   └── anthropic/       # Unique subdirectory matching package name
+│       ├── conftest.py
+│       └── test_client.py
+```
+
+Core package can use `tests/` directly with topic subdirectories:
+
+```
+packages/core/
+├── tests/
+│   ├── conftest.py
+│   ├── core/
+│   │   └── test_agents.py
+│   └── openai/
+│       └── test_client.py
+```
+
+## Fixture Guidelines
+
+- Use `conftest.py` for shared fixtures within a test directory
+- Before adding new fixtures, check if existing ones can be reused or extended
+- Use descriptive names: `mapper`, `test_request`, `mock_client`
+
+## File Naming
+
+- Files starting with `test_` are test files — do not use this prefix for helpers
+- Use `conftest.py` for shared utilities
+
+## Integration Tests
+
+Tests marked with `@skip_if_..._integration_tests_disabled` require:
+- `RUN_INTEGRATION_TESTS=true` environment variable
+- Appropriate API keys in environment or `.env` file
+
+## Best Practices
+
+- Run only related tests, not the entire suite
+- Review existing tests to understand coding style before creating new ones
+- Use print statements for debugging, then remove them when done
+- Resolve all errors and warnings before committing