Handle expired agreements and rewrite README

- Implement robust End User Agreement expiry detection and handling
- Add graceful error recovery for failed accounts
- Rewrite README.md to focus on user benefits
- Add documentation guidelines to AGENTS.md

specs/planning.md

@@ -13,6 +13,7 @@ Refactor the `bank2ff` application from a prototype script into a robust, testab
 - **Healer Strategy**: Detect and heal historical duplicates that lack external IDs.
 - **Dry Run**: Safe mode to preview changes.
 - **Rate Limit Handling**: Smart caching and graceful skipping to respect 4 requests/day limits.
+- **Robust Agreement Handling**: Gracefully handle expired GoCardless EUAs without failing entire sync.
 
 ## 2. Architecture