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

Move 18 graph subcommand handlers (594 lines) out of main.rs:
link, link-add, link-impact, link-audit, link-orphans,
triangle-close, cap-degree, normalize-strengths, differentiate,
trace, spectral-*, organize, interference.

main.rs: 3130 → 2518 lines.

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

poc-memory/src/cli/mod.rs

@@ -0,0 +1,6 @@
+// cli/ — command-line interface handlers
+//
+// Split from main.rs for readability. Each module handles a group
+// of related subcommands.
+
+pub mod graph;