Issue #3586284: Clarify context item scope form copy and layout.

Description

Testing instructions

1. Install ai_context per project page
2. Switch to this MR branch
3. Clear cache
4. Go to /admin/config/ai/context/items/add
5. Fill in dummy context info
6. Read the scope text
7. Test enabling global
8. Test disabling global
9. Summary badge should should up only when global isn't enabled (after saving)
10. Text should be as expected from the issue

Checklist

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have updated the MR description to include: Closes #1234
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description
• (n/a) I have updated documentation for any new or changed functionality

AI Compliance

Note

Check the one that best describes your usage, or leave all unchecked if AI was not significantly used.

• AI Assisted Code
  Mainly written by a human; AI used for autocomplete or partial generation under full human supervision.

• AI Generated Code
  Mainly generated by AI, reviewed and approved by a human before this MR was created.

• Vibe Coded
  Generated by AI and only functionally reviewed before this MR was created.

Closes #3586284