Add a SubAgent tool that lets the main model delegate multi-step read
and search tasks to a fast, inexpensive sub-agent. The sub-agent runs
autonomously with its own ConversationRuntime, making multiple tool
calls without round-tripping through the main model.
- SubAgent tool with prompt, task_type (Explore/Plan/Verify), and
optional model override
- Explore: read_file, glob_search, grep_search, WebFetch, WebSearch
- Plan: Explore + TodoWrite
- Verify: Plan + bash
- subagentModel config field in settings.json (falls back to main model)
- Uses ProviderRuntimeClient + SubagentToolExecutor (same as Agent tool)
This dramatically reduces token usage for information-gathering tasks.
Example: "find all Rust files that import X and summarize their usage"
→ one SubAgent call instead of 10 sequential main-model round trips.
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