GeezerVison

Co-authored-by: Hugo van Kemenade <[email protected]>

This is broken with cpython main which removed the color keyword argument
to HelperFormatter in python/cpython#142274

This is broken with cpython main which removed the color keyword argument
to HelperFormatter in python/cpython#142274

This is broken with cpython main which removed the color keyword argument
to HelperFormatter in python/cpython#142274

agentgrep runs on Python 3.15, which becomes the default interpreter
and the version CI tests. On 3.15 every command, `--version` included,
failed with `TypeError: HelpFormatter.__init__() got an unexpected
keyword argument 'color'`: the themed formatter forwarded `color=True`
to `argparse.HelpFormatter`, and 3.15 removed that keyword
(python/cpython#142274). The published release crashed the same way
under a bare `uvx agentgrep`, which uv 0.13 runs on 3.15 (#168).

- **Formatter fix**: the class `create_themed_formatter()` returns
  binds its theme as a class attribute instead of overriding
  `__init__`; argparse builds the formatter as
  `formatter_class(prog=...)` on 3.14 and 3.15 and applies color
  afterwards.

- **Tests**: `tests/test_help_color.py` renders `--help` with color
  forced on and off; it fails on 3.15 without the fix.

- **Python 3.15 default**: `.tool-versions` lists 3.15 first, the test
  matrix runs 3.15 in place of 3.14, and the release job builds on
  3.15.

- **Packaging**: the trove classifiers add Python 3.15, so PyPI lists
  it as supported.

- **Dependabot**: override `katex` to `^0.18.2` in the docs pnpm
  workspace for GHSA-238p-pmpm-9mq7 until mermaid-cli 12 brings katex
  ^0.18; mermaid still renders flowcharts and `$$` labels.

- **Changelog**: a Fixes entry, "agentgrep runs on Python 3.15".

Fixes #168