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/storage/migrations.go

@@ -3,6 +3,7 @@ package storage
 var migrations = []string{
 	divacodeSchema,
 	companionSchema,
+	addReasoningColumn,
 }
 
 const divacodeSchema = `
@@ -17,6 +18,7 @@ CREATE TABLE IF NOT EXISTS messages (
     conversation_id TEXT NOT NULL REFERENCES conversations(id),
     role            TEXT NOT NULL CHECK(role IN ('user','assistant','system','tool')),
     content         TEXT NOT NULL,
+    reasoning       TEXT,
     tool_call       TEXT,
     token_count     INTEGER,
     created_at      TEXT NOT NULL DEFAULT (datetime('now'))
@@ -111,3 +113,5 @@ CREATE TABLE IF NOT EXISTS agent_strategies (
     created_at  TEXT NOT NULL DEFAULT (datetime('now'))
 );
 `
+
+const addReasoningColumn = `ALTER TABLE messages ADD COLUMN reasoning TEXT;`