docs: document Solana app package

[sedge-bot]

Summary

• replaces the packages/app-sol create-next-app boilerplate README with Solana-specific RipGuard package documentation
• documents the SolSab/Sablier Lockup flow, wallet surfaces, env vars, scripts, and chain notes
• points contributors to the EVM app and repo-level README for broader project context

Verification

• README marker smoke: Solana-specific markers present; boilerplate markers absent
• corepack pnpm --filter app-sol test (140 passed)
• git diff --check -- packages/app-sol/README.md
• staged diff exposure scan for token/secret patterns

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