Handle plot and HTML outputs in btwExtra tools

[brianmsm]

Current btwExtra covers text outputs (run-R-code). We still need:

• Plot/graphics handling: capture base/ggplot output and return as image (e.g., base64 or temp file path) so MCP/LLM can view plots.
• HTML/table output handling: capture HTML widgets/gt/kableExtra output and return a viewable representation (HTML or PNG snapshot).

Scope ideas:

• Add a plot capture helper that runs code and saves last plot to PNG; extend run-r-code tool or add a new tool ().
• For HTML outputs (gt, htmlwidgets), capture the rendered HTML and provide a safe snapshot (HTML and, optionally, PNG for clients that can render images).
• Consider registration flags/annotations for read-only vs. file-writing behavior.

This issue tracks designing/implementing plot/HTML output support to complement the existing text-only behavior.

[brianmsm]

v0.2.0 is out with plot capture support:

• btwExtra_tool_env_run_r_code now captures plots as PNG (base64 + temp file path) and includes a media payload + path hint in the result.
• Guidance added to read the temp file via magick::image_read(<path>) when a user explicitly wants to view the plot.
• Descriptions now use markdown for better rendering in clients.