feat(subagents): add per-agent model overrides

[zaixiaziyang]

Summary

• Let each delegated agent select its own custom provider and model.
• Validate every explicit model before starting work and reject the whole batch atomically when one is unavailable.
• Keep readonly child registries from exposing unclassified MCP tools.

Main-conversation readonly enforcement is intentionally left to a separate PR so this change stays focused on subagent model selection and child hardening.

Testing

• Subagent frontend tests: 86 passed
• GUI typecheck and production build
• Targeted Biome check and git diff --check