Addresses #17313
Problem: The visual context meter in the status line was confusing and
continued to draw negative feedback, and context reporting should remain
an explicit opt-in rather than part of the default footer.
Solution: Remove the visual meter, restore opt-in context remaining/used
percentage items that explicitly say "Context", keep existing
context-usage configs working as a hidden alias, and update the setup
text and snapshots.
Eric Traut
470510174b