feat(phases4,7,8): implement Agent/ReAct, Code Execution, and Prompt Server

Completes Phase 4 (Agentic Loop with ReAct), Phase 7 (Code Execution),
and Phase 8 (Prompt Server) as specified in the implementation plan.

**Phase 4: Agentic Loop with ReAct Pattern (agent.rs - 398 lines)**
- Complete AgentExecutor with reasoning loop
- LlmResponse enum: ToolCall, FinalAnswer, Reasoning
- ReAct parser supporting THOUGHT/ACTION/ACTION_INPUT/FINAL_ANSWER
- Tool discovery and execution integration
- AgentResult with iteration tracking and message history
- Integration with owlen-agent CLI binary and TUI

**Phase 7: Code Execution with Docker Sandboxing**

*Sandbox Module (sandbox.rs - 255 lines):*
- Docker-based execution using bollard
- Resource limits: 512MB memory, 50% CPU
- Network isolation (no network access)
- Timeout handling (30s default)
- Container auto-cleanup
- Support for Rust, Node.js, Python environments

*Tool Suite (tools.rs - 410 lines):*
- CompileProjectTool: Build projects with auto-detection
- RunTestsTool: Execute test suites with optional filters
- FormatCodeTool: Run formatters (rustfmt/prettier/black)
- LintCodeTool: Run linters (clippy/eslint/pylint)
- All tools support check-only and auto-fix modes

*MCP Server (lib.rs - 183 lines):*
- Full JSON-RPC protocol implementation
- Tool registry with dynamic dispatch
- Initialize/tools/list/tools/call support

**Phase 8: Prompt Server with YAML & Handlebars**

*Prompt Server (lib.rs - 405 lines):*
- YAML-based template storage in ~/.config/owlen/prompts/
- Handlebars 6.0 template engine integration
- PromptTemplate with metadata (name, version, mode, description)
- Four MCP tools:
  - get_prompt: Retrieve template by name
  - render_prompt: Render with Handlebars variables
  - list_prompts: List all available templates
  - reload_prompts: Hot-reload from disk

*Default Templates:*
- chat_mode_system.yaml: ReAct prompt for chat mode
- code_mode_system.yaml: ReAct prompt with code tools

**Configuration & Integration:**
- Added Agent module to owlen-core
- Updated owlen-agent binary to use new AgentExecutor API
- Updated TUI to integrate with agent result structure
- Added error handling for Agent variant

**Dependencies Added:**
- bollard 0.17 (Docker API)
- handlebars 6.0 (templating)
- serde_yaml 0.9 (YAML parsing)
- tempfile 3.0 (temporary directories)
- uuid 1.0 with v4 feature

**Tests:**
- mode_tool_filter.rs: Tool filtering by mode
- prompt_server.rs: Prompt management tests
- Sandbox tests (Docker-dependent, marked #[ignore])

All code compiles successfully and follows project conventions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/owlen-tui/src/chat_app.rs

@@ -2815,7 +2815,6 @@ impl ChatApp {
             model: self.controller.selected_model().to_string(),
             temperature: Some(0.7),
             max_tokens: None,
-            max_tool_calls: 20,
         };
 
         // Get the provider
@@ -2834,18 +2833,18 @@ impl ChatApp {
         };
 
         // Create agent executor
-        let executor = AgentExecutor::new(provider, mcp_client, config, None);
+        let executor = AgentExecutor::new(provider, mcp_client, config);
 
         // Run agent
         match executor.run(user_message).await {
-            Ok(answer) => {
+            Ok(result) => {
                 self.controller
                     .conversation_mut()
-                    .push_assistant_message(answer);
+                    .push_assistant_message(result.answer);
                 self.agent_running = false;
                 self.agent_mode = false;
                 self.agent_actions = None;
-                self.status = "Agent completed successfully".to_string();
+                self.status = format!("Agent completed in {} iterations", result.iterations);
                 self.stop_loading_animation();
                 Ok(())
             }