feat: install local plugin dependencies from package.json

[elithrar]

Enables local plugins and custom tools to use external npm dependencies by running bun install after adding @opencode-ai/plugin.

• local plugins in .opencode/plugin/ previously could not use external npm packages. If a plugin imported a package like shescape, it would SILENTLY fail because dependencies were not installed and the tool would throw.
• we now also run bun install to install any additional dependencies defined in the config directory's package.json.

usage:

Add a package.json to your .opencode/ directory:

{
  "dependencies": {
    "shescape": "^2.1.0"
  }
}

[rekram1-node]

/review

[github-actions]

Suggestion: The new "Dependencies" section in plugins.mdx contradicts existing text at line 50 which states:

Local plugins are loaded directly from the plugin directory. Dependencies are not installed automatically. If your local plugin requires external packages, publish it to npm instead and add it to your config.

Since this PR enables local plugin dependencies, this old text should be updated to reflect the new behavior. Consider updating line 50 to remove the statement about dependencies not being installed automatically.

[elithrar]

Fixed — I rebased incorrectly on mobile after #6301.