[FEATURE]: Allow plugins to register slash commands

[nsoufian]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem

Plugin authors currently have no mechanism to interact with users through the OpenCode interface.

Limitations

• Plugins cannot register dynamic slash commands that appear in the command list
• Plugins cannot trigger plugin-managed processes from user input
• Plugins cannot update internal state through UI interactions

Impact

This limits what plugins can offer. Common use cases like toggling plugin modes, triggering syncs, or managing plugin configuration require workarounds outside the OpenCode interface.

Expected behavior

Plugins should be able to define slash commands that users can invoke directly, with the output persisted as session messages.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Plugin Hook for Instant TUI Commands #5305: [FEATURE]: Plugin Hook for Instant TUI Commands - Proposes allowing plugins to register instant commands that execute without agent involvement
• [FEATURE]: Plugin Hooks for spawning dialogs #9911: [FEATURE]: Plugin Hooks for spawning dialogs - Requests plugin hooks for creating custom modals and dialogs for native feeling interactions
• [FEATURE]: Plugin Support for Modal Human-in-the-loop Interactions  #5147: [FEATURE]: Plugin Support for Modal Human-in-the-loop Interactions - Proposes exposing modal infrastructure to plugins for interactive capabilities
• [FEATURE]: Generic UI Intent Channel for cross-client plugin-driven UX #6330: [FEATURE]: Generic UI Intent Channel for cross-client plugin-driven UX - Suggests a comprehensive UI intent system that would enable plugins to drive interactions

These issues explore similar concepts of extending plugin capabilities for user interaction. Feel free to ignore if your specific use case differs from these.