feat(config): verify DB connection before saving set-db

- Check connection with create_pool before writing to config
- Show 'Database connection failed' on error, do not overwrite config
- Update AGENTS.md and README.md

Made-with: Cursor

src/commands/config.rs

@@ -4,6 +4,12 @@ use anyhow::Result;
 pub async fn run(action: crate::ConfigAction) -> Result<()> {
     match action {
         crate::ConfigAction::SetDb { url } => {
+            // Verify connection before writing config
+            let pool = crate::db::create_pool(&url)
+                .await
+                .map_err(|e| anyhow::anyhow!("Database connection failed: {}", e))?;
+            drop(pool);
+
             let cfg = Config {
                 database_url: Some(url.clone()),
             };