fix(skill): reload slash skill content on invocation

[lazyayuan]

Issue for this PR

Closes #49451

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Skill-backed slash commands captured item.content when the Command service initialized, so editing an existing SKILL.md had no effect until OpenCode restarted. This changes the slash command template getter to call Skill.require() on each invocation, reloading the current disk-backed content while keeping command discovery metadata cached.

How did you verify your code works?

Verified manually in the desktop app:

1. Invoked an existing project Skill through its slash command.
2. Edited and saved its SKILL.md in VS Code.
3. Invoked the same slash command again in the same OpenCode process.
4. Confirmed the second invocation used the updated Skill content without restarting.

Screenshots / recordings

Not applicable; this changes command content loading and has no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR