v0.9.0

Added

• Add max_subagent_depth with a default of 5.
• A subagent call can continue an earlier child session.
• Add a single strategy that offers one subagent tool for all subagents.

Changed

• Secret prompts now mask each character, to show that a paste arrived.
• A subagent result now contains the child session id.
• defer_tools now covers subagent tools.
• Subagent references in the manifest use the agent.<id> form.
• Session cancel now stops running subagents.
• Rename sub_agents to subagents in the manifest.