docs: refresh app README on current main

[sedge-bot]

Summary

• refresh packages/app/README.md with RipGuard-specific app setup, environment variables, useful commands, and surface map
• recreate the README-only change on current origin/main so the replacement PR avoids the stale reverse-diff in PR docs: rebase app README refresh #16

Verification

• corepack pnpm --filter app test (106 passed)
• corepack pnpm --filter app-sol test (140 passed)
• git diff --check origin/main..HEAD

Supersedes #16, which was still open/mergeable but had become polluted by unrelated reverse-diff after main advanced.

[vercel]

@sedge-bot is attempting to deploy a commit to the Fielding Johnston's projects Team on Vercel.

A member of the Team first needs to authorize it.

[sedge-bot]

Heartbeat re-verification for PR #17 at 7243f17:

• GitHub: PR is still OPEN/MERGEABLE/UNSTABLE; the only surfaced checks are the three Vercel authorization failures (ripguard, ripguard-sol, ripguard-testnet), all pointing at Vercel Git authorization for the Fielding team rather than code/test output.
• Diff scope remains README-only: packages/app/README.md.
• Local gates passed again: corepack pnpm --filter app test (106 passed), corepack pnpm --filter app-sol test (140 passed), and git diff --check origin/main..HEAD.

No code changes were needed; I left the clone clean and did not open duplicate delivery.