feat(amp): add model mapping support for routing unavailable models to alternatives

- Add AmpModelMapping config to route models like 'claude-opus-4.5' to 'claude-sonnet-4'
- Add ModelMapper interface and DefaultModelMapper implementation with hot-reload support
- Enhance FallbackHandler to apply model mappings before falling back to ampcode.com
- Add structured logging for routing decisions (local provider, mapping, amp credits)
- Update config.example.yaml with amp-model-mappings documentation

config.example.yaml

@@ -55,6 +55,28 @@ quota-exceeded:
 # When true, enable authentication for the WebSocket API (/v1/ws).
 ws-auth: false
 
+# Amp CLI Integration
+# Configure upstream URL for Amp CLI OAuth and management features
+#amp-upstream-url: "https://ampcode.com"
+
+# Optional: Override API key for Amp upstream (otherwise uses env or file)
+#amp-upstream-api-key: ""
+
+# Restrict Amp management routes (/api/auth, /api/user, etc.) to localhost only (recommended)
+#amp-restrict-management-to-localhost: true
+
+# Amp Model Mappings
+# Route unavailable Amp models to alternative models available in your local proxy.
+# Useful when Amp CLI requests models you don't have access to (e.g., Claude Opus 4.5)
+# but you have a similar model available (e.g., Claude Sonnet 4).
+#amp-model-mappings:
+#  - from: "claude-opus-4.5"       # Model requested by Amp CLI
+#    to: "claude-sonnet-4"         # Route to this available model instead
+#  - from: "gpt-5"
+#    to: "gemini-2.5-pro"
+#  - from: "claude-3-opus-20240229"
+#    to: "claude-3-5-sonnet-20241022"
+
 # Gemini API keys (preferred)
 #gemini-api-key:
 #  - api-key: "AIzaSy...01"