plugin: configured local plugin file paths silently dropped since #46105

Summary

Since #46105 (2026-08-30), configured plugin entries that resolve to an absolute file path are silently dropped with only a level=WARN log line ("configured plugin path must be a directory"), while the plugins docs still document file paths as valid config entries. Upgrading silently disables local plugins.

Environment

• opencode version: v2 branch (source build at 71779ae5de, channel local)
• OS: Darwin 25.6.0 (darwin arm64)
• Terminal: Unavailable (headless server context)
• Shell: /bin/zsh
• Install/channel: source build (channel local), v2 line
• Active plugins: two local plugins configured by absolute path (see Reproduction)

Reproduction

1. Configure a local plugin by absolute file path:

   {
     "plugins": [
       "/Users/me/opencode-plugin/plugin.js"
     ]
   }
   

   where plugin.js has a default export (export default { id, setup }). This worked before 2026-08-30.
2. Start opencode serve (or any server that loads plugins).
3. The plugin is not loaded. The server log contains:

   level=WARN message="configured plugin path must be a directory" target=/Users/me/opencode-plugin/plugin.js
   

   No error is surfaced to the user; the plugin simply disappears (its models, tools, and skills are gone).

Expected Behavior

Either:

• Configured file paths keep working (resolve the file directly as the plugin entrypoint, as before #46105), or
• The docs are updated to remove file-path examples, and the drop is surfaced loudly (config error / visible warning) instead of a WARN log line — silently losing plugins on upgrade is the worst outcome.

Actual Behavior

packages/core/src/config/plugin/source.ts (since 6a2c3e91c7, #46105) drops any configured target that fs.isFile with the warning "configured plugin path must be a directory" and returns Option.none, so the plugin is never loaded. This also affects relative file paths (../shared/plugin.ts) and file:// entries pointing at files, which resolve to absolute paths before the check.

Additional Context

• The docs still show file paths as valid: packages/www/src/docs/content/plugins.mdx examples include "../shared/plugin.ts", "/absolute/path/plugin.ts", and "file:///home/me/plugins/local". config.mdx says "Load plugins from packages or local plugin directories" and only shows directory examples.
• Directory targets only work when the directory contains an index.ts/index.js (PluginSourceDirectory.entrypoint); a directory without one is also dropped ("configured plugin directory has no index entrypoint").
• Workaround: point the config at the plugin directory and add an index.js re-exporting the plugin file. Package (npm/Git) plugin entries are unaffected because they resolve through package.json exports/main.
• This bit in production on a self-hosted instance: both configured plugins (a subscription auth plugin and a pipeline plugin) silently vanished after rebasing onto the v2 branch that includes #46105; models provided by the subscription plugin disappeared from the catalog.