feat: add dynamic Bedrock model discovery

Add automatic discovery of AWS Bedrock models using ListFoundationModels API.
When AWS credentials are detected, models that support streaming and text output
are automatically discovered and made available.

- Add @aws-sdk/client-bedrock dependency
- Add discoverBedrockModels() with caching (default 1 hour)
- Add resolveImplicitBedrockProvider() for auto-registration
- Add BedrockDiscoveryConfig for optional filtering by provider/region
- Filter to active, streaming, text-output models only
- Update docs/bedrock.md with auto-discovery documentation

package.json

@@ -147,6 +147,7 @@
   "packageManager": "pnpm@10.23.0",
   "dependencies": {
     "@agentclientprotocol/sdk": "0.13.0",
+    "@aws-sdk/client-bedrock": "^3.975.0",
     "@buape/carbon": "0.14.0",
     "@clack/prompts": "^0.11.0",
     "@grammyjs/runner": "^2.0.3",