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>

.env.example

@@ -9,11 +9,6 @@
 # - webkit: Safari engine
 # - msedge: Microsoft Edge
 # PLAYWRIGHT_BROWSER=firefox
-#
-# PLAYWRIGHT_HEADLESS: Run browser without visible window
-# - true: Browser runs in background, saves CPU (default)
-# - false: Browser opens a visible window (useful for debugging)
-# PLAYWRIGHT_HEADLESS=true
 
 # Extra Read Paths (Optional)
 # Comma-separated list of absolute paths for read-only access to external directories.
@@ -25,56 +20,17 @@
 # Google Cloud Vertex AI Configuration (Optional)
 # To use Claude via Vertex AI on Google Cloud Platform, uncomment and set these variables.
 # Requires: gcloud CLI installed and authenticated (run: gcloud auth application-default login)
-# Note: Use @ instead of - in model names (e.g., claude-opus-4-5@20251101)
+# Note: Use @ instead of - in model names for date-suffixed models (e.g., claude-sonnet-4-5@20250929)
 #
 # CLAUDE_CODE_USE_VERTEX=1
 # CLOUD_ML_REGION=us-east5
 # ANTHROPIC_VERTEX_PROJECT_ID=your-gcp-project-id
-# ANTHROPIC_DEFAULT_OPUS_MODEL=claude-opus-4-5@20251101
+# ANTHROPIC_DEFAULT_OPUS_MODEL=claude-opus-4-6
 # ANTHROPIC_DEFAULT_SONNET_MODEL=claude-sonnet-4-5@20250929
 # ANTHROPIC_DEFAULT_HAIKU_MODEL=claude-3-5-haiku@20241022
 
 # ===================
-# Alternative API Providers
+# Alternative API Providers (GLM, Ollama, Kimi, Custom)
 # ===================
-# NOTE: These env vars are the legacy way to configure providers.
-# The recommended way is to use the Settings UI (API Provider section).
-# UI settings take precedence when api_provider != "claude".
-
-# Kimi K2.5 (Moonshot) Configuration (Optional)
-# Get an API key at: https://kimi.com
-#
-# ANTHROPIC_BASE_URL=https://api.kimi.com/coding/
-# ANTHROPIC_API_KEY=your-kimi-api-key
-# ANTHROPIC_DEFAULT_SONNET_MODEL=kimi-k2.5
-# ANTHROPIC_DEFAULT_OPUS_MODEL=kimi-k2.5
-# ANTHROPIC_DEFAULT_HAIKU_MODEL=kimi-k2.5
-
-# GLM/Alternative API Configuration (Optional)
-# To use Zhipu AI's GLM models instead of Claude, uncomment and set these variables.
-# This only affects AutoForge - your global Claude Code settings remain unchanged.
-# Get an API key at: https://z.ai/subscribe
-#
-# ANTHROPIC_BASE_URL=https://api.z.ai/api/anthropic
-# ANTHROPIC_AUTH_TOKEN=your-zhipu-api-key
-# API_TIMEOUT_MS=3000000
-# ANTHROPIC_DEFAULT_SONNET_MODEL=glm-4.7
-# ANTHROPIC_DEFAULT_OPUS_MODEL=glm-4.7
-# ANTHROPIC_DEFAULT_HAIKU_MODEL=glm-4.5-air
-
-# Ollama Local Model Configuration (Optional)
-# To use local models via Ollama instead of Claude, uncomment and set these variables.
-# Requires Ollama v0.14.0+ with Anthropic API compatibility.
-# See: https://ollama.com/blog/claude
-#
-# ANTHROPIC_BASE_URL=http://localhost:11434
-# ANTHROPIC_AUTH_TOKEN=ollama
-# API_TIMEOUT_MS=3000000
-# ANTHROPIC_DEFAULT_SONNET_MODEL=qwen3-coder
-# ANTHROPIC_DEFAULT_OPUS_MODEL=qwen3-coder
-# ANTHROPIC_DEFAULT_HAIKU_MODEL=qwen3-coder
-#
-# Model recommendations:
-# - For best results, use a capable coding model like qwen3-coder or deepseek-coder-v2
-# - You can use the same model for all tiers, or different models per tier
-# - Larger models (70B+) work best for Opus tier, smaller (7B-20B) for Haiku
+# Configure alternative providers via the Settings UI (gear icon > API Provider).
+# The Settings UI is the recommended way to switch providers and models.