feat(plugin): fall back to OpenCode-stored /connect credentials for plugin auth (#17)

The plugin's health check and /v1/models discovery only read options.apiKey
or LITELLM_API_KEY / LITELLM_MASTER_KEY, so a key stored via OpenCode's
/connect command (~/.local/share/opencode/auth.json) was invisible to it:
a key-only proxy failed the health check with a 401 and silently skipped
discovery. Fall back to the stored credential (precedence: options.apiKey >
env var > auth.json, read once per process) and write the resolved key back
into provider options so real chat completions authenticate too. Updates
README and CHANGELOG under [Unreleased].