Add daily rates social poster

[Sikkra]

Summary

• add scripts/daily_post.ts to build daily net-APY social posts from existing pool rate data
• post to X and Farcaster with env-only credentials (X_BEARER_TOKEN, NEYNAR_API_KEY, NEYNAR_SIGNER_UUID)
• isolate channel failures so missing or failing credentials are logged without crashing the run
• add dry-run npm entrypoint and handle current Soroban symbol XDR values in the shared decoder

Closes #92

Verification

• npm run daily-post:dry-run generated 3 readable X/Farcaster-safe posts, all <= 280 chars
• npm run daily-post without credentials exited 0 and logged missing X/Farcaster credentials per channel without crashing
• git diff --check