Bug Description

The pluginAutoInstall config field exists in the schema but has no effect. Users can set pluginAutoInstall: false in their config file, but plugins still auto-install because the config value is never checked by any code.

Only the undocumented OPENCODE_DISABLE_PLUGIN_INSTALL=true environment variable actually disables auto-install, but there's no way to set this from the config file.

Steps to Reproduce

1. Add "pluginAutoInstall": false to opencode.json
2. Start opencode
3. Observe that plugins are still auto-installed (check ~/.cache/opencode/packages/)

Root Cause

• npm.ts: add() and install() never check any disable flag
• config.ts: pluginAutoInstall schema field is not wired to set the env var
• flag.ts: No OPENCODE_DISABLE_PLUGIN_INSTALL flag defined as a runtime getter

Expected Behavior

Setting pluginAutoInstall: false in config should prevent automatic plugin installation, equivalent to setting OPENCODE_DISABLE_PLUGIN_INSTALL=true.

Impact

This caused a critical issue in our deployment: opencode auto-installed an npm package that overwrote our patched fork plugin, destroying session compression state and losing data.