Support `{file:...}` / `{env:...}` variable interpolation for usage plugin secrets

[bogorad]

The usage plugin reads config/auth values as literal strings, so secret file references are not resolved. This makes it hard to use secret managers such as sops-nix where keys are exposed as files in /run/secrets/....

Current behavior

• usage-config.jsonc fields (for example apiKey) are parsed as plain JSONC and used directly.
• Auth entries used by providers are consumed as raw key / access strings.
• Values like {file:/run/secrets/proxy_api_key} or {env:PROXY_API_KEY} are not expanded.

Expected behavior

Support the same variable interpolation style as OpenCode config:

• {env:VAR_NAME}
• {file:/absolute/or/relative/path}

Why this matters

Many setups (including NixOS + sops-nix) store secrets as files under /run/secrets/.... Without interpolation support, users must keep/duplicate secrets in plain text config, a big no-no for some setups.

Reference (OpenCode config behavior)

• https://opencode.ai/docs/config#variables
• https://opencode.ai/docs/config#files