[FEATURE]: exec variable substitution

[nikaro]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

I would like to be able to do exec variables substitution, example for retrieving credentials from op (1Password CLI):

{
  "$schema": "https://opencode.ai/config.json",
  "provider": {
    "anthropic": {
      "models": {},
      "options": {
        "apiKey": "{exec:op read 'op://Vault/Provider/token'}"
      }
    }
  }
}

Could be interesting for these issues:

• https://github.com/anomalyco/opencode/issues/5423
• https://github.com/anomalyco/opencode/issues/4318

Looks like a previous attempt at something like that was done: https://github.com/anomalyco/opencode/pull/2405