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/llama/client.go

@@ -15,12 +15,13 @@ type Client struct {
 }
 
 type CompletionRequest struct {
-	Prompt      string  `json:"prompt"`
-	Temperature float64 `json:"temperature"`
-	TopP        float64 `json:"top_p"`
-	MaxTokens   int     `json:"n_predict"`
-	Stream      bool    `json:"stream"`
-	CachePrompt bool    `json:"cache_prompt"`
+	Prompt      string   `json:"prompt"`
+	Temperature float64  `json:"temperature"`
+	TopP        float64  `json:"top_p"`
+	MaxTokens   int      `json:"n_predict"`
+	Stop        []string `json:"stop,omitempty"`
+	Stream      bool     `json:"stream"`
+	CachePrompt bool     `json:"cache_prompt"`
 }
 
 type CompletionResponse struct {