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

cmd/divad/main.go

@@ -96,7 +96,7 @@ func main() {
 		cancel()
 	}()
 
-	p := tea.NewProgram(channel.NewTUIModel(ag), tea.WithAltScreen())
+	p := tea.NewProgram(channel.NewTUIModel(ag), tea.WithAltScreen(), tea.WithMouseCellMotion())
 	if _, err := p.Run(); err != nil {
 		slog.Error("tui error", "error", err)
 		os.Exit(1)