feat: Phase 2 — chat templates, reasoning parser, collapsible TUI, mouse support

- internal/llama/template.go: ChatML/Llama3 template engine with stop tokens
- internal/agent/reasoning.go: parse <think>/<reasoning> tags from LLM output
- companion/promptbuilder.go: output structured BuildResult, add date/time context
- agent/agent.go: wire templates + reasoning split into message loop
- storage: add reasoning TEXT column to messages table + migration
- channel/tui.go: collapsible reasoning (tab toggle), mouse scroll, polished dimmed style
- config: CHAT_TEMPLATE env var (chatml|llama3|none)
- plan.md: mark Phase 2 complete, add themes TODO

pkg/types/types.go

@@ -15,6 +15,7 @@ type Message struct {
 	ID        string    `json:"id"`
 	Role      Role      `json:"role"`
 	Content   string    `json:"content"`
+	Reasoning string    `json:"reasoning,omitempty"`
 	ToolCall  *ToolCall `json:"toolCall,omitempty"`
 	CreatedAt time.Time `json:"createdAt"`
 }
@@ -32,6 +33,19 @@ type Conversation struct {
 	UpdatedAt time.Time `json:"updatedAt"`
 }
 
+type SegmentType string
+
+const (
+	SegSystem    SegmentType = "system"
+	SegUser      SegmentType = "user"
+	SegAssistant SegmentType = "assistant"
+)
+
+type PromptSegment struct {
+	Type    SegmentType
+	Content string
+}
+
 type Mood string
 
 const (