Improve support for accessing Open Editors in AI Agents

[CamilleLetavernier]

What it does

• Add two distinct mechanisms to support "Open Editors":
  • A prompt fragment (currently enabled by default in Universal, Architect and Coder) that references the existing variable "openEditors" (or #_ff)
    • automatic integration of the #_ff variable
    • can be added or removed from any agent via prompt customization
  • A contribution that automatically adds open editors to the current context, guarded by a user preference (enabled by default)
    • works for any agent
    • immediate user feedback via the context files
    • user can remove individual files

Both mechanisms are currently overlapping, as they are both enabled by default. I added both so we can more easily compare, but we might want to choose one (or keep both but only enable one or the other by default).

How to test

• Default prompt behavior:
  • Open the AI Chat
  • Open some files in Theia
  • Remove them from context (or disable the preference Auto Add Open Editors)
  • Ask Architect, Coder or Universal about open files: they should at least be able to list them
• Auto Add to Context:
  • Open the AI Chat
  • Open some files in Theia: they should automatically be added to the context (and can then be manually removed by the user)
  • Talk to any agent other than Architect, Coder and Universal: they should be able to reference these files

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
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