feat(ai-tools): drop OpenCode and Grok Build

- opencode.json / grok.json leave the built-in catalog; opencode-usage.js,
  its poller, transcript branch and table creation go with it (existing
  tables and session rows stay as history).
- A Grok payload reaching the Claude hook route (Grok runs the Claude hooks
  itself) is dropped instead of recorded as tool = 'grok'.
- quota.js no longer reads Grok or OpenCode Go logins.
- Redone on v2.8.22 from c6b2892 (feat/grok-usage, never merged).

server/__tests__/fixtures/ai-tools/septool.json

@@ -1,7 +1,7 @@
 {
-  "id": "grok",
-  "label": "Grok",
-  "detect": ["grok"],
+  "id": "septool",
+  "label": "Separator tool",
+  "detect": ["septool"],
   "prompt": {
     "mode": "argv",
     "separator": "--"