prompt improvement: get 100 buffer lines above and below instead of whole file

[ataha322]

Note

Low Risk
Low risk: only changes prompt context generation for visual selections by limiting included buffer text, with minimal impact beyond prompt contents and potential off-by-one range edge cases.

Overview
Updates the visual_selection prompt to embed only nearby buffer context (100 lines above/below the selection) instead of dumping the entire file.

Adds get_surrounding_context(range, n) to compute the bounded slice of buffer lines used in the prompt, reducing prompt size and improving relevance.

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[ThePrimeagen]

fix pweeze

[ilyaZar]

wouldnt this compute from = start_row - 100 which becomes negative when the visual selection is near the top of a file

iam not a pro but I think its worht checking if nvim treats negative nvim_buf_get_lines() indexes as relative tod the end of the buffer not as "clamp to line 1" ; so top-of-file selections in larger files can return unrelated middle/end slices or {} instead of the intended surrounding context

see #171 fro my try to rework this a bit