feat(installer): offer CodeGraph Pro beta signup after install and upgrade (#1297)

One-time, strictly opt-in prompt at the end of codegraph install and
codegraph upgrade to join the CodeGraph Pro beta waitlist (same list as
the getcodegraph.com homepage form). Nothing is sent unless the user
answers yes AND enters an email; either answer is recorded machine-wide
so no later install or upgrade re-asks, and --yes / non-interactive / CI
runs never see the prompt.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/bin/codegraph.ts

@@ -2394,6 +2394,10 @@ program
         warn: (m: string) => warn(m),
         error: (m: string) => error(m),
         platform: process.platform,
+        offerBetaSignup: async () => {
+          const { maybeOfferBetaSignup } = await import('../installer/beta-signup');
+          await maybeOfferBetaSignup({ source: 'cli-upgrade' });
+        },
       }
     );
     process.exit(code);