fix(context): reference file-backed bufs with non-empty buftype

accidental side-effect of `last_used_valid_win` and `get_filename`; I
didn't realize help windows have a special buftype but still an
associated file. anyway, we no longer need those guards because now we
store a Context _before_ opening input and select windows.
(I think... might be missing a low-risk edge-case)

relevant to https://github.com/nickjvandyke/opencode.nvim/issues/247
closes https://github.com/nickjvandyke/opencode.nvim/issues/248