Previously, double-click word selection in markdown used simple
space-based boundaries, so `foo.bar()` would be selected as a single
word.

Now we use `CharClassifier` which properly handles word boundaries:

- **Regular markdown text**: Punctuation (`.`, `(`, `)`, etc.) now acts
as word boundaries. Double-clicking on `foo.bar()` selects just `foo` or
`bar`.
- **Code blocks**: Uses the language's word characters. For example, in
JavaScript, `$foo` and `#bar` are selected as whole words since `$` and
`#` are configured as word characters.

Release Notes:

- Improved double-click word selection in Agent Panel to respect
punctuation and language-specific word characters.