feat(runtime): defer agent tools

[Astro-Han]

Summary

• add a reusable deferred agent tool group for agent_spawn, agent_list, and agent_output
• wire desktop tool availability so agent tools load through load_tools({ group: 'agent' })
• cover default hidden surface, same-turn guard, loaded use, and unchanged local-read child toolset semantics

Closes#52 when the broader issue accepts this slice.

Verification

• npm run -w @maka/runtime test
• npm run -w @maka/headless test
• npm run typecheck
• git diff --check