Add integration testing

[fogfish]

Problem: Only unit tests exist, no integration tests with mock LLM.

Required Changes:

1. Create testing/mock_llm.go:

// MockLLM provides a fake LLM for testing
type MockLLM struct {
    responses []string
    index     int
}

func NewMockLLM(responses ...string) *MockLLM

func (m *MockLLM) Prompt(ctx context.Context, msgs []chatter.Message, opts ...chatter.Opt) (*chatter.Reply, error) {
    // Return canned responses
}

2. Create agent/integration_test.go with full agent flow tests
3. Test all three agent types end-to-end
4. Test error conditions
5. Test multi-turn conversations

Estimated Effort: 3 hours
Skills Required: Testing, mock design