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/agent/reasoning.go

@@ -0,0 +1,41 @@
+package agent
+
+import (
+	"strings"
+)
+
+type ParsedResponse struct {
+	Reasoning string
+	Content   string
+}
+
+func ParseResponse(raw string) ParsedResponse {
+	tags := []struct {
+		open  string
+		close string
+	}{
+		{"<reasoning>", "</reasoning>"},
+		{"<reason>", "</reason>"},
+		{"<thinking>", "</thinking>"},
+		{"<think>", "</think>"},
+		{"", ""},
+	}
+
+	for _, t := range tags {
+		if t.open == "" {
+			break
+		}
+		idxOpen := strings.Index(raw, t.open)
+		idxClose := strings.LastIndex(raw, t.close)
+		if idxOpen != -1 && idxClose != -1 && idxClose > idxOpen {
+			reasoning := raw[idxOpen+len(t.open) : idxClose]
+			content := raw[:idxOpen] + raw[idxClose+len(t.close):]
+			return ParsedResponse{
+				Reasoning: strings.TrimSpace(reasoning),
+				Content:   strings.TrimSpace(content),
+			}
+		}
+	}
+
+	return ParsedResponse{Content: strings.TrimSpace(raw)}
+}