feat(core): add plugin-registered slash commands to the command list

[nsoufian]

Closes #10262

What does this PR do?

Currently, plugin authors have no way to interact with users through the OpenCode interface. Plugins cannot:

• Register dynamic slash commands that appear in the command list
• Trigger plugin-managed processes from user input
• Update plugin internal state through UI interactions

This PR addresses these limitations by introducing plugin-registered slash commands.

How it works

Plugins can now define commands that appear in the slash command list. When invoked, these commands execute plugin-defined logic and persist the output as session messages—no LLM involvement required.

Example

import type { Plugin } from "@opencode-ai/plugin"

export const TogglePlugin: Plugin = async () => {
  return {
    command: [
      {
        name: "toggle",
        description: "toggle plugin state",
        hints: ["$ARGUMENTS"],
        mode: "plugin",
      },
    ],
    "command.execute": async (input) => {
      if (input.command !== "toggle") return undefined
      return {
        parts: [{ type: "text", text: `toggle:${input.arguments.trim() || "off"}` }],
      }
    },
  }
}

Benefits

• User interaction: Plugins can respond to user intent through slash commands
• State management: Plugin state can be updated via UI without external tooling
• Process triggers: Plugins can kick off background processes on demand
• Discoverability: Plugin commands appear alongside built-in commands
• Clean architecture: Plugin command execution is isolated in a dedicated service

How did you verify your code works?

• Automated: Unit tests for command listing and execution (bun test --cwd packages/opencode test/command/plugin-commands.test.ts)
• Manual: Created a plugin using the example above, verified the command appears in the list, invoked /toggle on, and confirmed output persists in the session

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found a potentially related PR:

Potential Duplicate:

• PR feat(opencode) plugin commands #7563: feat(opencode) plugin commands - feat(opencode) plugin commands #7563

This PR appears to address the same core feature: adding plugin command registration. It's referenced multiple times across different search queries for plugin command functionality.

I'd recommend reviewing PR #7563 to understand if this is an earlier attempt at the same feature that may have been closed, superseded, or needs rebasing against the current dev branch.