Run Cypher queries from the rdx console

[paracycle]

Goal

Let rdx console run Cypher directly, addressing the "query mode" idea from #868's review.

Stacked on #868 (base uk_add_cypher_query_engine).

What it does

On top of the normal IRB session (with graph in scope for Ruby, e.g. graph["Foo"]), it registers two commands:

rubydex> query MATCH (n:Class|Module) RETURN n.name ORDER BY n.name
rubydex> schema

The query command takes the rest of the line verbatim, so the Cypher doesn't need to be valid Ruby or quoted. Rather than swapping the REPL's evaluator for a modal toggle, Cypher lines are simply prefixed with query — the clean, idiomatic IRB realization of a query mode.

Notes

• Console logic is extracted into lib/rubydex/console.rb; the query path lives in Console.run_query so it's testable without driving IRB.
• IRB stays a soft dependency: it's a default gem, so it's not declared in the gemspec. The commands register only when the installed IRB exposes the command API (>= 1.13); on older IRB the console degrades to a plain Ruby session.
• Currently uses Query#render (string output). Once an object-returning Query#run lands, query could return real handle objects instead of a printed table.

Verification

rake ruby_test TEST=test/console_test.rb (7 tests), rubocop clean, live console smoke test.