feat(cli): restructure CLI commands for better UX

- Add top-level `multica login` that combines auth + workspace auto-discovery
- Restructure daemon into subcommands: start, stop, status, logs
- Add background daemon mode with PID management
- Add daemon deregistration on shutdown (new API endpoint + SQL query)
- Remove unused commands: runtime list, status, agent get/delete/stop
- Make `config` show config directly instead of requiring `config show`
- Update README to reflect new CLI structure

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/pkg/db/queries/runtime.sql

@@ -39,3 +39,8 @@ UPDATE agent_runtime
 SET status = 'online', last_seen_at = now(), updated_at = now()
 WHERE id = $1
 RETURNING *;
+
+-- name: SetAgentRuntimeOffline :exec
+UPDATE agent_runtime
+SET status = 'offline', updated_at = now()
+WHERE id = $1;