feat(config): add fallback_providers with local Ollama terminal fallback

Adds explicit fallback chain to the root config template.
This ensures local-first fallback is configured out of the box,
addressing the risk of external AI dependency without degradation path.

- Primary: anthropic (as configured)
- Terminal: ollama/qwen3:30b (sovereign local fallback)

Closes #483

config.yaml

@@ -1,6 +1,19 @@
 model:
   default: claude-opus-4-6
   provider: anthropic
+fallback_providers:
+  # Primary chain entry — mirrors model.provider above.
+  - provider: anthropic
+    model: claude-opus-4-6
+    timeout: 120
+    reason: Primary provider (Anthropic)
+  # Sovereign terminal fallback — always last.
+  - provider: ollama
+    model: qwen3:30b
+    base_url: http://localhost:11434/v1
+    timeout: 300
+    reason: Terminal fallback — local Ollama (sovereign, no API cost)
+
 toolsets:
 - all
 agent: