feat(execution): Add response parser for LLM artifact extraction

Implements regex-based response parser to extract file creation actions
and artifacts from LLM text responses. Agents can now produce actual
work products (files, PRs) instead of just returning instructions.

Changes:
- pkg/ai/response_parser.go: New parser with 4 extraction patterns
  * Markdown code blocks with filename comments
  * Inline backtick filenames followed by "content:" and code blocks
  * File header notation (--- filename: ---)
  * Shell heredoc syntax (cat > file << EOF)

- pkg/execution/engine.go: Skip sandbox when SandboxType empty/none
  * Prevents Docker container errors during testing
  * Preserves artifacts from AI response without sandbox execution

- pkg/ai/{ollama,resetdata}.go: Integrate response parser
  * Both providers now parse LLM output for extractable artifacts
  * Fallback to task_analysis action if no artifacts found

- internal/runtime/agent_support.go: Fix AI provider initialization
  * Set DefaultProvider in RoleModelMapping (prevents "provider not found")

- prompts/defaults.md: Add Rule O for output format guidance
  * Instructs LLMs to format responses for artifact extraction
  * Provides examples and patterns for file creation/modification
  * Explains pipeline: extraction → workspace → tests → PR → review

Test results:
- Before: 0 artifacts, 0 files generated
- After: 2 artifacts extracted successfully from LLM response
- hello.sh (60 bytes) with correct shell script content

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

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

internal/runtime/agent_support.go

@@ -386,9 +386,30 @@ func (r *SharedRuntime) executeBrief(ctx context.Context, assignment *council.Ro
 	// Create execution engine
 	engine := execution.NewTaskExecutionEngine()
 
-	// Create AI provider factory
+	// Create AI provider factory with proper configuration
 	aiFactory := ai.NewProviderFactory()
 
+	// Register the configured provider
+	providerConfig := ai.ProviderConfig{
+		Type:         r.Config.AI.Provider,
+		Endpoint:     r.Config.AI.Ollama.Endpoint,
+		DefaultModel: "llama3.1:8b",
+		Timeout:      r.Config.AI.Ollama.Timeout,
+	}
+
+	if err := aiFactory.RegisterProvider(r.Config.AI.Provider, providerConfig); err != nil {
+		r.Logger.Warn("⚠️  Failed to register AI provider: %v", err)
+	}
+
+	// Set role mapping with default provider
+	// This ensures GetProviderForRole() can find a provider for any role
+	roleMapping := ai.RoleModelMapping{
+		DefaultProvider:  r.Config.AI.Provider,
+		FallbackProvider: r.Config.AI.Provider,
+		Roles:            make(map[string]ai.RoleConfig),
+	}
+	aiFactory.SetRoleMapping(roleMapping)
+
 	engineConfig := &execution.EngineConfig{
 		AIProviderFactory:  aiFactory,
 		MaxConcurrentTasks: 1,