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/sqlite.go

@@ -29,7 +29,12 @@ func Open(path string) (*DB, error) {
 
 func (db *DB) Migrate() error {
 	for _, m := range migrations {
-		if err := db.Exec(m); err != nil {
+		err := db.Exec(m)
+		if err != nil && m == addReasoningColumn {
+			// may already exist on fresh DBs
+			continue
+		}
+		if err != nil {
 			return err
 		}
 	}