Bottom toolbar format string for customization

[rolandwalker]

Description

Like the prompt, the bottom toolbar can be customized, using the same format strings, with a special format string \B to represent the standard toolbar (in the first position only).

When \B is included the user's customizations appear on the second line.

When \B is not included, the user may override the toolbar completely. Transient notices will still appear to the right of the user's format.

Like the prompt, both a CLI option --toolbar and a ~/.myclirc option are provided, with the CLI option taking precedence.

Motivation: useful information takes up repeated space in the prompt, but needs to be displayed only once in the toolbar.

Example:

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Medium: Missing regression tests for the new toolbar formatting paths.
   The PR adds multiple branches with behavior differences (\B prefix, full override, dynamic notice placement, cached rendering while typing) but no tests exercise them, so regressions are likely to slip in.
   Relevant code: mycli/clitoolbar.py:67, mycli/main.py:1506

   Suggested tests:

   • toolbar='\B\d' keeps default toolbar on line 1 and renders custom content on line 2.
   • toolbar='\d' fully overrides default toolbar, but transient/dynamic notices still append on the right.
   • toolbar='\B' keeps current default behavior (no extra custom line).
   • get_custom_toolbar() caching behavior while buffer is non-empty vs empty.

No correctness or security defects stood out in the implementation itself.

[scottnemes]

Verified toolbar customization works as expected