feat: convert tmux monitoring into a plugin (B10)

drizzle/sqlite/0036_classy_nighthawk.sql

@@ -0,0 +1,5 @@
+-- tmux_session_tags moved to the tmux-monitor plugin, whose first migration
+-- renames it to p_tmux_monitor_session_tags. Dropping it here would lose
+-- every saved tag, so this only stops drizzle tracking it. SELECT 1 because
+-- MySQL rejects an empty query.
+SELECT 1;