docs(concepts): update default model to claude-opus-4-7 (PR #743)

- google-adk.mdx: update comparison table LangGraph example from
  anthropic:claude-sonnet-4-6 → anthropic:claude-opus-4-7
- quickstart.mdx: add callout noting claude-opus-4-7 as the new
  default for workspaces that don't pin a model explicitly

Pairs with molecule-core PR #743 / closes #727.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

content/docs/google-adk.mdx

@@ -196,7 +196,7 @@ The Google ADK adapter exposes live session transcripts to the canvas "look over
 
 ```yaml title="LangGraph workspace"
 runtime: langgraph
-model: anthropic:claude-sonnet-4-6
+model: anthropic:claude-opus-4-7
 ```
 
 ```yaml title="AutoGen workspace"

content/docs/quickstart.mdx

@@ -12,6 +12,10 @@ using the bundled `molecule-dev` template.
 - Go 1.25+ and Node 20+ if building from source
 - An LLM API key (Claude, OpenRouter, or Gemini)
 
+<Callout type="info">
+  **Default model:** New workspaces that do not specify a model use `anthropic:claude-opus-4-7` by default. To pin a different model, set `model: <provider>:<name>` in the workspace `config.yaml` or pass `MODEL_DEFAULT` in your `.env`.
+</Callout>
+
 ## Option A: One-command start (recommended)
 
 ```bash