docs: plugins page is outdated for TUI plugins, jsonc configs, and install behavior #21409

Description

The /docs/plugins page is out of date relative to the current plugin implementation.

A few parts of the page are currently incorrect or incomplete:

• it does not clearly distinguish server plugins from TUI plugins
• it does not document opencode.jsonc as an alternative to opencode.json
• it does not explain what happens when both .json and .jsonc are present in the same directory
• it implies local plugin directory auto-loading applies more broadly than it actually does
• it does not accurately describe how npm plugin targets are split between opencode.json[c] and tui.json[c]
• install / upgrade / cache wording is outdated

This makes it easy to misconfigure plugins, especially when working with local TUI plugins or trying to understand server-vs-TUI plugin package entrypoints.

Plugins

No response

OpenCode version

Current docs / dev

Steps to reproduce

1. Open https://opencode.ai/docs/plugins
2. Compare the page with the current plugin implementation
3. Notice the mismatch around server vs TUI plugins, opencode.jsonc, and install / upgrade behavior

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response