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

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Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>
Co-authored-by: Yara <yara@zed.dev>