opencode run --agent should fail closed on unknown names

Summary

opencode run --agent NAME does exact-key lookup. After a plugin remaps config keys to display names, the original key is not found. Unknown names silently use default_agent instead of failing.

Repro

1. Plugin registers the agent under display name Sisyphus - ultraworker and hides the config key sisyphus.
2. opencode run --agent sisyphus ping

Actual

Default agent runs. Easy to miss in e2e (every --agent foo appears to pass as Sisyphus).

Expected

Unknown agent names error with the list of registered names. Optionally resolve case-insensitive / alias maps plugins can provide.

Suggested fix

Fail closed on unknown --agent. Allow plugins to register aliases (config key to registered name).