Issue for this PR

Closes #32954

Type of change

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

What does this PR do?

Before this change, choosing a skill from /skills immediately inserted a single /<skill> command into the prompt. That made it hard to combine skills for one request.

Now the selector supports toggling multiple skills with space, then applying them with enter.

• Selected skills are shown with [x].
• The selector title shows the selected count.
• Pressing enter without toggling anything still keeps the previous single-select behavior.

This also teaches command prompt handling to consume chained leading skill commands, for example:

/effect /investigate debug this failure

Only slash commands that resolve to skills are consumed as additional skill context. Unknown or non-skill slash commands are left as normal prompt text to preserve existing behavior.

If you paste a large clearly AI generated description here your PR may be IGNORED or CLOSED!

How did you verify your code works?

• bun test test/session/prompt.test.ts --timeout 30000
• bun typecheck
• Pre-push bun turbo typecheck
• Manually tested the TUI /skills flow:
  • opened the skills selector
  • selected multiple skills with space
  • verified the selected count and [x] markers
  • applied the selection with enter
  • verified the composer received chained skill commands

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

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