docs: fix Example user journey to match the actual UI

[smartcalsystems-sketch]

Problem

The README's "Example user journey" described steps that don't match the actual UI:

• Step 1 implied /connect syncs a GitHub repository; it only links GitHub login + a Stellar wallet.
• Step 2 implied a dashboard flow to mark an issue with a reward; no such bounty-creation UI exists yet.
• Step 3 implied sponsor funding happens at /dashboard/sponsor; funding actually happens on the issue's own detail page at /issues/[id], per the Feature map table earlier in the README.

Fix

Rewrote the journey to match the documented feature map (step 2 now describes funding at /issues/[id], the dashboard-marking step is removed), and added the missing repo-sync / bounty-creation UI as a Roadmap item instead of describing it as already shipped.

Closes #690

[vercel]

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