feat: ChatGPT Custom GPT support in chat bot + sqcp SMTP routing fix

- chat.ts: system prompt now includes step-by-step ChatGPT Custom GPT
  setup (openapi.json import + OAuth), Claude/Cursor/Windsurf config,
  and mortgage broker guidance — bot no longer incorrectly says ChatGPT
  is unsupported
- smtp.ts: all sqcp_* accounts now route to mail.squaremcp.com (SQCP_SMTP_HOST)
  instead of the fetcherpay server
- tools.ts: ACCOUNT_PARAM description now lists all 14 mailboxes including
  the 7 squaremcp.com accounts so Claude picks the right one without guessing
- package.json: postinstall hook runs imapflow patch script after npm install
- hermes-k8s.yaml: updated image digest to current production build

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

package.json

@@ -15,7 +15,8 @@
     "test:product-site:cleanup": "node product/site/cleanup-test-submissions.mjs",
     "deploy:product-site:verify": "bash product/site/deploy-and-verify.sh",
     "test": "vitest run",
-    "test:watch": "vitest"
+    "test:watch": "vitest",
+    "postinstall": "node scripts/patch-imapflow.cjs"
   },
   "dependencies": {
     "@anthropic-ai/sdk": "^0.96.0",