cli: extract node commands from main.rs into cli/node.rs

Move 15 node subcommand handlers (310 lines) out of main.rs:
render, write, used, wrong, not-relevant, not-useful, gap,
node-delete, node-rename, history, list-keys, list-edges,
dump-json, lookup-bump, lookups.

main.rs: 2518 → 2193 lines.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

poc-memory/src/cli/mod.rs

@@ -4,3 +4,4 @@
 // of related subcommands.
 
 pub mod graph;
+pub mod node;