Skill tool error messages don't suggest alternatives on unknown skill

Problem

When the Skill tool fails to find a skill, the error message provides no diagnostic information:

Unknown skill: commit

This gives zero signal about why it failed or how to fix it.

Expected Behavior

The error should include:

1. Fuzzy match suggestions - "Did you mean commit-commands:commit?"
2. Partial matches - Show skills containing the search term
3. Namespace hint - If the skill exists in a namespace, suggest the full path

Example

Current:

<error>Unknown skill: commit</error>

Expected:

<error>
Unknown skill: commit

Did you mean one of these?
  - commit-commands:commit
  - commit-commands:commit-push-pr
  - commit-commands:clean_gone

Tip: Plugin skills use format "plugin-name:skill-name"
</error>

Context

• Plugin commit-commands is installed with skills: commit, commit-push-pr, clean_gone
• Invoking Skill with skill: "commit" fails silently
• The correct invocation is skill: "commit-commands:commit"
• User has no way to discover this without manually inspecting ~/.claude/plugins/

Impact

This creates a poor developer experience - users may assume a skill doesn't exist when it's just namespaced differently. Debugging requires manual filesystem inspection.