feat(external-llm): standardize tiers and optimize model selection

- Rename tiers: opus/sonnet/haiku → frontier/mid-tier/lightweight
- Align with industry benchmarks (MMLU, GPQA, Chatbot Arena)
- Add /external command for LLM mode control
- Fix invoke.py timeout passthrough (now 600s default)

Tier changes:
- Promote gemini-2.5-pro to frontier (benchmark-validated)
- Demote glm-4.7 to mid-tier then removed (unreliable)
- Promote gemini-2.5-flash to mid-tier

New models added:
- gpt-5-mini, gpt-5-nano (GPT family coverage)
- grok-code (Grok/X family)
- glm-4.5-air (lightweight GLM)

Removed (redundant/unreliable):
- o3 (not available)
- glm-4.7 (timeouts)
- gpt-4o, big-pickle, glm-4.5-flash (redundant)

Final: 11 models across 3 tiers, 4 model families

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/README.md

@@ -28,6 +28,7 @@ Slash commands for quick actions. User-invoked (type `/command` to trigger).
 | `/programmer` | | Code development tasks |
 | `/gcal` | `/calendar`, `/cal` | Google Calendar access |
 | `/usage` | `/stats` | View usage statistics |
+| `/external` | `/llm`, `/ext` | Toggle and use external LLM mode |
 
 ### Kubernetes (`/k8s:*`)