Add ESLint rule to enforce Lingui msg usage

Adds a custom ESLint rule 'lingui-msg-rule' that ensures the Lingui _()
function is called with msg`` template literals or plural/select macros,
preventing accidental misuse like _('string') which bypasses i18n.

https://claude.ai/code/session_01JMXXPUgAHiSBGmfGwUojKy

eslint.config.mjs

@@ -121,6 +121,7 @@ export default defineConfig(
       ],
       'bsky-internal/use-exact-imports': 'error',
       'bsky-internal/use-prefixed-imports': 'error',
+      'bsky-internal/lingui-msg-rule': 'error',
 
       /**
        * React & React Native