Multi-account support: credentials keyed by account name

- prompts.json now has account field per prompt
- bot.js looks up env vars by account prefix (e.g. THEJUNIPERSKY_USER)
- .env restructured with account-prefixed credentials
- Adding a new Twitter account = new env vars + new prompt blocks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prompts.json

@@ -12,7 +12,8 @@
         20
       ],
       "minDays": 2
-    }
+    },
+    "account": "thejunipersky"
   },
   {
     "name": "personality",
@@ -28,6 +29,7 @@
         3
       ],
       "minGapHours": 4
-    }
+    },
+    "account": "thejunipersky"
   }
 ]