refactor: make Settings UI the single source of truth for API provider

Remove legacy env-var-based provider/mode detection that caused misleading
UI badges (e.g., GLM badge showing when Settings was set to Claude).

Key changes:
- Remove _is_glm_mode() and _is_ollama_mode() env-var sniffing functions
  from server/routers/settings.py; derive glm_mode/ollama_mode purely from
  the api_provider setting
- Remove `import os` from settings router (no longer needed)
- Update schema comments to reflect settings-based derivation
- Remove "(configured via .env)" from badge tooltips in App.tsx
- Remove Kimi/GLM/Ollama/Playwright-headless sections from .env.example;
  add note pointing to Settings UI
- Update CLAUDE.md and README.md documentation to reference Settings UI
  for alternative provider configuration
- Update model IDs from claude-opus-4-5-20251101 to claude-opus-4-6
  across registry, client, chat sessions, tests, and UI defaults
- Add LEGACY_MODEL_MAP with auto-migration in get_all_settings()
- Show model ID subtitle in SettingsModal model selector
- Add Vertex passthrough test for claude-opus-4-6 (no date suffix)

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

server/services/process_manager.py

@@ -346,7 +346,7 @@ class AgentProcessManager:
 
         Args:
             yolo_mode: If True, run in YOLO mode (skip testing agents)
-            model: Model to use (e.g., claude-opus-4-5-20251101)
+            model: Model to use (e.g., claude-opus-4-6)
             parallel_mode: DEPRECATED - ignored, always uses unified orchestrator
             max_concurrency: Max concurrent coding agents (1-5, default 1)
             testing_agent_ratio: Number of regression testing agents (0-3, default 1)