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

internal/companion/engine.go

@@ -45,7 +45,7 @@ func NewEngine(cfg *config.Config, db *storage.DB) *Engine {
 	}
 }
 
-func (e *Engine) BuildPrompt(ctx context.Context, input *PromptInput) (string, error) {
+func (e *Engine) BuildPrompt(ctx context.Context, input *PromptInput) (*BuildResult, error) {
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
@@ -54,7 +54,7 @@ func (e *Engine) BuildPrompt(ctx context.Context, input *PromptInput) (string, e
 	mood := e.mood.Current()
 	observations := e.reflection.RecentObservations(5)
 
-	prompt := e.promptBuilder.Build(&BuildInput{
+	result := e.promptBuilder.Build(&BuildInput{
 		SystemPrompt: e.cfg.SystemPrompt,
 		Traits:       traits,
 		Relationship: rel,
@@ -66,7 +66,7 @@ func (e *Engine) BuildPrompt(ctx context.Context, input *PromptInput) (string, e
 	})
 
 	e.lastContact = time.Now()
-	return prompt, nil
+	return result, nil
 }
 
 func (e *Engine) WriteDiary(ctx context.Context) error {