Clarify agent model selectors

[cbusillo]

Summary

• keep agent.create.models as the stable tool field after live harness validation
• tighten the tool schema description to call entries agent/model selector slugs
• rename the prompt guide heading without adding a larger explanation block

Validation

• code exec --json --sandbox read-only --max-seconds 240 ... live harness: GPT-5.5 called agent.create with models: ["antigravity"], Antigravity launched, and returned AGENT_OK
• cargo test -p code-core antigravity_receives_workspace_add_dir -- --nocapture
• cargo test -p code-core agent_tool -- --nocapture
• git diff --check
• ./build-fast.sh

Refs #99
Refs #134