feat: show upstream Git changes in Neovim

[jiminu]

Neovim now shows commits waiting to push or pull beside the branch and marks incoming changes in NvimTree after an external fetch. For example, a diverged branch displays ↑1↓2; existing files show ↓M, ↓D, or ↓R, and parent folders indicate incoming additions that are not yet present locally.

• Use matching solid commit arrows in Starship and Neovim for readability, with consistent color meanings: outgoing green, incoming red; tree modifications yellow, additions/renames green, deletions red. Hide zero counts and keep local Git badges. Reuse the tree highlight foregrounds in lualine while preserving its theme background.
• Read cached upstream refs asynchronously, refreshing every five seconds and on editor events. Recalculate counts and file differences only when refs change; never fetch, including lazy object downloads in partial clones.
• Register the new module as managed configuration and cover fetched/diverged refs, detached HEAD, missing upstream, worktrees, separate tree roots, partial clones, and rendering with the approved plugins.

Validation:

• TMPDIR=/private/tmp bash scripts/check.sh passed for the initial feature commit; the final glyph/color refinement was checked with the focused tests and color smoke below.
• Focused Neovim configuration and Git remote tests with the installed, approved NvimTree/lualine plugins passed after the solid-arrow change.
• Starship TOML parsing and actual Git status rendering passed with ↑1↓2 in a private HOME.
• Dark/light rendering smoke check confirms joined glyphs, shared foregrounds and preserved statusline backgrounds, including custom highlight changes. An interactive demo was also checked for the initial feature.

New remote-only files appear in the tree after pulling; their parent directories carry an incoming indicator beforehand. Partial clones may show deletion/addition instead of a rename when similarity detection needs unavailable blobs.