feat(llm): add Amazon Bedrock as a first-class provider

Bedrock uses the Converse API (langchain-aws) and the AWS credential chain, so
it has its own client like Anthropic/Google rather than the OpenAI-compatible
registry. langchain-aws is an optional dependency (pip install ".[bedrock]"),
lazy-imported with a clear install hint; importing the package never requires
it. The model name is a Bedrock model ID / inference profile ID.

.env.example

@@ -21,6 +21,12 @@ NVIDIA_API_KEY=
 # optional (local servers need none).
 #OPENAI_COMPATIBLE_API_KEY=
 
+# AWS Bedrock (provider "bedrock", install with: pip install ".[bedrock]").
+# Auth uses the standard AWS credential chain; set the region (and optionally a
+# named profile). No single API key.
+#AWS_DEFAULT_REGION=us-west-2
+#AWS_PROFILE=
+
 # Optional: point at a remote Ollama server. When unset, defaults to
 # the local instance at http://localhost:11434/v1. Convention follows
 # the broader Ollama ecosystem; both the CLI dropdown and programmatic