[FEATURE]: Add /skills command to list and quick-invoke skills

[nguyenngothuong]

Feature hasn't been suggested before.

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

Related but different from #7716 (marketplace discovery) and #7533 (display in sidebar).

Problem

Currently, there's no quick way to:

1. List available skills from the TUI prompt
2. Invoke a specific skill directly via slash command

Users must either:

• Know skill names beforehand and hope the LLM picks them up
• Run opencode debug skill in terminal (not discoverable)

Proposed Solution

1. /skills command - List all available skills

/skills

Shows a searchable list of loaded skills with:

• Skill name
• Short description
• Source (global/project)

2. /skill:<name> - Quick invoke syntax

/skill:code-review
/skill:meeting-notes

When typing /skill:, autocomplete shows available skills (similar to @ for files).

This would work like Claude Code's new skill invocation feature.

Use Cases

• Discoverability: New users can explore what skills are available
• Quick access: Power users can invoke skills without typing full prompts
• Consistency: Matches existing patterns (/models, /themes, /sessions)

Suggested Implementation

1. Add /skills to command list (similar to /models)
2. Register skill: as autocomplete trigger (like @ for files)
3. When selected, inject skill name into system prompt or trigger skill loading

Environment

• OpenCode version: v1.0.x
• This is a feature request for TUI

---

Happy to contribute a PR if the maintainers agree with this direction!