nvim: add async git status markers to telescope file pickers

[phlip9-github-agent]

Summary

• add nvim/lua/git_file_status.lua for async repo-wide file status
  collection and caching keyed by (repo_root, diff_base)
• honor g:gitgutter_diff_base (fallback HEAD) and include
  staged/unstaged/committed-vs-base plus untracked files
• add nvim/lua/telescope_git_file_status.lua wrappers for find_files
  and buffers to render +/-/~ markers and refresh asynchronously
• wire O, <space>O, and T keymaps to the wrapped pickers
• add unit/integration tests for parsing/reduction/cache and telescope
  attachment/display behavior
• keep design docs in module headers to avoid drift

Testing

• just nvim-test lua/test/git_file_status_spec.lua
• just nvim-test lua/test/telescope_git_file_status_spec.lua
• just nvim-test lua/test/telescope_diff_outline_spec.lua