feat: implement agent executor with tool calling, sub-agents, and comprehensive test suite

- Add AgentExecutor implementing LLM-based agentic loop with tool calling, max iterations, and stop conditions
- Introduce agent preset tools (bash, file_exists, http_get, run_module, etc.) with extensible registry pattern
- Add sub-agent spawning capability via spawn_agent tool call with recursive depth limits and validation
- Implement ToolExecutor for custom tool execution with template rendering and error handling
- Add agent session persistence and memory management with sliding window configuration
- Create comprehensive E2E test suite covering 15+ agent workflow scenarios (minimal, custom tools, planning, multi-goal, structured output, tracing hooks, file tools, orchestration, Python tools, sub-agents, nested sub-agents, and validation)
- Add agent-and-llm test data directory with 17 YAML workflow fixtures
- Update integration tests to include agent workflow directories
- Add AgentTool and AgentConfig types with validation for duplicate names and unknown presets
- Implement LLM streaming test utilities
- Update documentation (CLAUDE.md, HACKING.md, README.md) with agent features and CLI examples

test/testdata/workflows/agent-and-llm/test-agent-python-tools.yaml

@@ -0,0 +1,29 @@
+kind: module
+name: test-agent-python-tools
+description: Test agent with exec_python and exec_python_file preset tools
+tags: test,agent,python
+
+params:
+  - name: target
+    required: true
+    default: example.com
+
+steps:
+  - name: agent-python
+    type: agent
+    log: "Running Python tools agent for {{Target}}"
+    query: "Use Python to parse and analyze data about {{Target}}. Write a small Python script to check if the target is a valid domain."
+    system_prompt: "You are a Python-powered analyst. Use exec_python for inline code and exec_python_file for scripts."
+    max_iterations: 5
+    agent_tools:
+      - preset: exec_python
+      - preset: exec_python_file
+      - preset: save_content
+      - preset: bash
+    exports:
+      python_result: "{{agent_content}}"
+
+  - name: verify-python-tools
+    type: bash
+    log: "Verifying Python tools agent output"
+    command: echo "Python tools result - {{python_result}}"