feat(cli): add read-only list and show subcommands

tapir list — table of stored summaries (date, title|id, channel, AI,
fallback), recent-first. tapir show <video-id> — full summary with
highlights and takeaways. DSN + user id from TAPIR_DB_DSN/TAPIR_USER_ID,
never hardcoded. main.go gains a minimal os.Args[1] dispatcher kept flat
so Worker F's auth/run cases union cleanly at merge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/tapir/main.go

@@ -1,10 +1,41 @@
-// Command tapir is the service entrypoint. Scaffold: it identifies itself so
-// the CI smoke test has something to grep for, and exits. Wiring the HTTP
-// server, watcher, adapters, and config is part of the build.
+// Command tapir is the service entrypoint and CLI. Subcommands are dispatched off
+// os.Args[1]; each lives in its own file (list.go, show.go, …). The switch is kept
+// deliberately flat so concurrently-added subcommands union cleanly.
 package main
 
-import "fmt"
+import (
+	"context"
+	"fmt"
+	"os"
+)
 
 func main() {
-	fmt.Println("tapir: scaffold — not yet implemented")
+	if len(os.Args) < 2 {
+		usage()
+		os.Exit(2)
+	}
+
+	ctx := context.Background()
+	var err error
+	switch os.Args[1] {
+	case "list":
+		err = runList(ctx, os.Args[2:])
+	case "show":
+		err = runShow(ctx, os.Args[2:])
+	default:
+		usage()
+		os.Exit(2)
+	}
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "tapir:", err)
+		os.Exit(1)
+	}
+}
+
+func usage() {
+	fmt.Fprintln(os.Stderr, "usage: tapir <command> [args]")
+	fmt.Fprintln(os.Stderr, "commands:")
+	fmt.Fprintln(os.Stderr, "  list [-limit N]   list stored summaries, recent first")
+	fmt.Fprintln(os.Stderr, "  show <video-id>   show one summary in full")
 }