feat(tui): no-arg gem-contribute launches the Rooibos TUI

[cdhagmann]

Per OPEN_QUESTIONS Q4: bare gem-contribute (no subcommand) launches the Rooibos TUI when a Gemfile.lock is in cwd. Falls back to a clear error when no Gemfile.lock is found.

Scope:

• Modify lib/gem_contribute/cli.rb so no-argument invocation launches the TUI when ./Gemfile.lock exists
• When no Gemfile.lock: print "no Gemfile.lock found" and the USAGE
• All other subcommand invocations still dispatch to CLI verbs (no TUI)

Sub-issue of #2.

See: ROADMAP Phase 3, OPEN_QUESTIONS Q4