[FEATURE]: Add content-only configuration authority for embedded runtimes

Feature hasn't been suggested before.

• I verified that this exact feature has not been suggested before.

Describe the enhancement you want to request

I run OpenCode as a child process and pass the whole config through
OPENCODE_CONFIG_CONTENT. I expected that to be the final config, but it
isn't. Account config, managed files or preferences, and discovered config
directories are loaded later, so they can change the provider or add plugins.

Could there be an opt-in flag that treats OPENCODE_CONFIG_CONTENT as the only
config source? In that mode OpenCode would skip the usual config discovery and
dependency install. It shouldn't touch existing config files, and launches
without the flag should behave as they do now.

#43227 is close, but --bare intentionally keeps managed configuration and
only covers opencode run. #21264 only disables global config. I'm looking for
the same kind of isolation when OpenCode is embedded as a child process.