docs: RULES.md lists only the RPC endpoint as contacted by default (closes #755)

The External Communication checklist named CoinDesk and Blockscout as
always contacted. USD prices now come from Chainlink feeds read through
the RPC endpoint, and Blockscout and Etherscan stay off until the user
turns them on. Both lines now say so, in the wording sneak approved on
the issue.

Model: opus-5-5

RULES.md

@@ -22,10 +22,12 @@ contradicts either, the originals govern.
 
 ## External Communication
 
-- [ ] Extension contacts exactly three external services: configured RPC
+- [ ] Until the user changes a setting, the extension contacts only the
-      endpoint, CoinDesk price API, and Blockscout block-explorer API
+      configured RPC endpoint
 - [ ] No analytics, telemetry, or tracking
-- [ ] No user-specific data sent except to the configured RPC endpoint
+- [ ] No user-specific data sent except to the configured RPC endpoint, unless
+      the user turns on "Use Blockscout" or "Check recipients on Etherscan",
+      both off by default
 - [ ] No Infura/Alchemy hard dependency
 - [ ] No backend servers operated by the developer
 - [ ] RPC endpoint is user-configurable (defaults to publicnode.com)