Describe the feature or problem you’d like to solve

When selecting skills to install via gh skill search, the agent selection prompt only allows a single choice:

? Select target agent: Claude Code

This is inconsistent with gh skill install (without --agent), which already supports selecting multiple agents at once via prompter.MultiSelect.

Proposed solution

Change the agent selection in promptInstall (pkg/cmd/skills/search/search.go) from prompter.Select to
prompter.MultiSelect, matching the behavior of gh skill install:

? Select target agent(s): [space to select]
> [x] GitHub Copilot
  [ ] Claude Code
  [ ] Cursor
  [ ] Codex

The implementation involves two changes:

1. Replace opts.Prompter.Select("Select target agent:", ...) with opts.Prompter.MultiSelect("Select target agent(s):", ...)
   (mirroring install.go line ~877)
2. Update the exec.Command loop to iterate over all selected agents, since --agent currently accepts a single value

Additional context

• gh version: 2.92.0
• Affected file: pkg/cmd/skills/search/search.go, function promptInstall
• Reference: pkg/cmd/skills/install/install.go already uses MultiSelect for agent selection in its interactive flow