This PR adds the foundation for the subagents feature: - Add `SubagentsFeatureFlag` with staff-disabled default - Create `SubagentTool` struct with input schema for task/summary/context-low prompts - Register `SubagentTool` conditionally when feature flag is enabled - Tool returns stub message 'not yet implemented' for now Release Notes: - N/A --------- Co-authored-by: Amp <amp@ampcode.com> Co-authored-by: Mikayla Maki <mikayla@zed.dev> Co-authored-by: Yara <yara@zed.dev>