feat: expose skills as slash commands

[POBIM]

Summary

• Add skill loading to Command.list() so skills appear as /skill-name commands
• Skill commands invoke the skill tool to show 'Loaded skill: X' in UI
• Add (Skill) suffix in autocomplete dropdown to distinguish from MCP commands
• Fix skill tool error message to show actual skill names instead of array indices
• Regenerate SDK types with skill?: boolean field

What does this PR do?
Allows users to invoke skills directly via slash commands (e.g., /my-skill) instead of manually calling the skill tool. When a skill command is invoked, it automatically loads the skill and follows its instructions.
Before: Users had to know about the skill tool and call it manually
After: Skills appear in autocomplete with (Skill) suffix and can be invoked like any other command

How did you verify your code works?

• Created test skills in .opencode/skill/ directory
• Verified skills appear in TUI autocomplete with (Skill) suffix
• Confirmed /skill-name invocation triggers the skill tool and shows "Loaded skill: X" in UI
• Tested with arguments: /skill-name some arguments passes arguments correctly

[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:

I found a potentially related PR:

PR #9324: feat: add /skills command and skill autocomplete
#9324

This PR appears to address similar functionality - adding skill commands and autocomplete for skills. Both PRs are focused on exposing skills as executable commands with autocomplete support. This could be a duplicate or closely related work that may need coordination to avoid conflicts.