docs: put Sepolia in scope and write down the provider flag exception (closes #165)

The README's 1.0 non-goal said Ethereum mainnet only, while Sepolia
ships as a selectable network. It now puts mainnet and Sepolia in scope
and other chains out of it.

The injected provider's flag named after the other wallet stays, as an
interface-compatibility flag many dApps check. It is now written down
as an exception to the rule against naming competitors, in the README's
Policies section and in RULES.md, and the comment beside it says so.
Neither document spells the name, so script/check-censored, which
already allows the flag only in src/content/inpage.js and its built
copies, is unchanged.

Model: opus-5-5

RULES.md

@@ -118,4 +118,7 @@ contradicts either, the originals govern.
 - [ ] "Address" not "account" or "derived key"
 - [ ] "Password" not "encryption key" or "vault passphrase"
 - [ ] Error messages are full sentences
-- [ ] No competitor mentioned by name in code or documentation
+- [ ] No competitor mentioned by name in code or documentation. Written
+      exception: the injected provider in `src/content/inpage.js` sets the flag
+      named after the other wallet to `true`, an interface-compatibility flag
+      many dApps check (README.md, Policies)