feat: sync PROVIDER_NAMES with config schema and update README docs

Extract MODEL_PROVIDERS const from config schema as single source of truth
for provider names. PROVIDER_NAMES in TUI commands now imports from schema
instead of maintaining a hardcoded list. Adds tests verifying sync.

Updates README TUI Commands section with /model hot-swap documentation,
supported providers, and runtime model switching examples.

README.md

@@ -185,19 +185,46 @@ pnpm tui:fs
 | Command | Description |
 |---------|-------------|
 | `/help` | Show help |
+| `/model` | Show all model tiers and which is active |
+| `/model <tier>` | Switch active tier (`local`, `default`, `fast`, `complex`, or aliases `ollama`, `sonnet`, `haiku`, `opus`) |
+| `/model <tier> <provider/model>` | Hot-swap a tier's provider and model at runtime |
+| `/backend [provider]` | Show or switch local backend (`ollama`, `llamacpp`) |
+| `/login [provider]` | Authenticate with GitHub (OAuth device flow) |
 | `/reset` | Clear history |
 | `/status` | Show session info |
 | `/compact` | Compact conversation context |
 | `/usage` | Show token usage and cost |
 | `/verbose` | Toggle verbose output mode |
-| `/local` | Switch to local model |
-| `/cloud` | Switch to cloud model |
-| `/model` | Show model info and options |
 | `/pair` | Generate/list/revoke DM pairing codes |
 | `/fullscreen` | Switch to fullscreen mode |
-| `/transfer telegram` | Transfer session to Telegram |
+| `/transfer <dest>` | Transfer session to another frontend |
 | `/quit` | Exit |
 
+#### Runtime Model Switching
+
+Switch providers and models on the fly without editing config or restarting:
+
+```bash
+# Show current tiers
+/model
+
+# Switch active tier
+/model fast
+/model complex
+
+# Hot-swap a tier's provider/model
+/model default anthropic/claude-sonnet-4
+/model default zhipuai/glm-4.7
+/model fast github/gpt-4o-mini
+/model local ollama/glm-4.7-flash
+```
+
+The provider name must match a supported provider (`anthropic`, `openai`, `gemini`, `ollama`, `llamacpp`, `openrouter`, `bedrock`, `github`, `zhipuai`, `xai`). Tab completion is available for both tiers and provider names.
+
+For cloud Zhipu models, ensure `ZHIPUAI_API_KEY` is set or `api_key` is configured in the relevant tier.
+
+**Note:** The `/model` command works in the TUI only. WebChat sessions inherit the active tier from the daemon.
+
 ## Running as Service
 
 ```bash