reorganize subcommands into logical groups

60+ flat subcommands grouped into:
- Core (daily use): search, render, write, history, tail, status, query, used, wrong, gap
- Node: delete, rename, list, edges, dump
- Journal: write, tail, enrich
- Graph: link, audit, spectral, etc.
- Agent: daemon, knowledge-loop, consolidate, digest, etc.
- Admin: init, health, fsck, import, export, etc.

Also: remove dead migration code (migrate.rs, Migrate/JournalTsMigrate commands),
update memory-search and poc-hook for new subcommand paths, update daemon systemd
template for `agent daemon` path.

poc-memory/src/agents/daemon.rs

@@ -1690,7 +1690,7 @@ After=default.target
 
 [Service]
 Type=simple
-ExecStart={exe} daemon
+ExecStart={exe} agent daemon
 Restart=on-failure
 RestartSec=30
 Environment=HOME={home}
@@ -1748,7 +1748,7 @@ After=default.target
 
 [Service]
 Type=simple
-ExecStart={exe} daemon
+ExecStart={exe} agent daemon
 Restart=on-failure
 RestartSec=10
 Environment=HOME={home}