docs: add backend switch command design

docs/plans/2026-02-05-backend-switch-design.md

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+# Backend Switch Command Design
+
+**Date:** 2026-02-05
+**Status:** Approved
+
+## Overview
+
+Add `/backend` slash command to switch between local LLM providers (ollama, llamacpp) at runtime with lazy instantiation.
+
+## Config Schema Extension
+
+Extend `models` with optional `local_providers` object:
+
+```yaml
+models:
+  local:
+    provider: ollama
+    model: qwen2.5:14b
+    endpoint: http://localhost:11434
+
+  local_providers:
+    llamacpp:
+      endpoint: http://localhost:8080
+      # auth_token: optional
+```
+
+- `local` - Active local provider (backwards compatible)
+- `local_providers` - Additional provider configs keyed by name
+
+## Command
+
+```
+/backend              Show current local provider
+/backend ollama       Switch to ollama
+/backend llamacpp     Switch to llamacpp
+```
+
+### Command Type
+
+```typescript
+| { type: 'backend'; provider?: string }
+```
+
+## ModelRouter Changes
+
+```typescript
+// Swap local client at runtime
+setLocalClient(client: ModelClient, providerName: string): void
+
+// Get current local provider name for display
+getLocalProviderName(): string | undefined
+```
+
+## Flow
+
+1. User runs `/backend llamacpp`
+2. Command parsed, handler receives `{ type: 'backend', provider: 'llamacpp' }`
+3. Handler looks up `config.models.local_providers.llamacpp`
+4. If not found → error "Backend 'llamacpp' not configured"
+5. Instantiate `LlamaCppClient` with config
+6. Call `router.setLocalClient(client, 'llamacpp')`
+7. Report "Switched to llamacpp"
+
+## Error Handling
+
+| Scenario | Response |
+|----------|----------|
+| `/backend` (no arg) | "Local backend: ollama" |
+| Unknown provider | "Unknown backend 'foo'. Available: ollama, llamacpp" |
+| Not configured | "Backend 'llamacpp' not configured in local_providers" |
+| Server down | Fail on first chat (existing error handling) |
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `src/config/schema.ts` | Add `local_providers` to modelsSchema |
+| `src/models/router.ts` | Add `setLocalClient()`, `getLocalProviderName()` |
+| `src/frontends/tui/commands.ts` | Add backend command parsing |
+| `src/frontends/tui/minimal.ts` | Handle backend command |
+| `src/daemon/index.ts` | Store config, add client factory function |