Bind a (abandon) on the Neovim status buffer

[jennings]

Part of #57

Goal

Make abandon available on the Neovim status buffer to match VSCode and Emacs.

Current state

• VSCode magit profile: a → badjuju.abandon.cursor on both status and log
• Emacs: a → badjuju-abandon on both status and log
• Neovim magit profile: a → JJAbandon on log only — missing on status

Change

In clients/neovim/lua/badjuju/keymap.lua magit profile, add a → JJAbandon to the status.jujutsu buffer keymap.

Docs

• docs/src/clients/neovim.md — add a to the status keybinding table
• docs/src/reference/status-buffer.md — confirm Neovim column shows the binding

Tests

• clients/neovim/tests/keymap_spec.lua — assert a is bound on status.jujutsu

Verification

• redo fmt && redo check && redo test — all pass
• Open a .jj repo in Neovim, press a on a commit in the status buffer, confirm it abandons

[jennings]

Already implemented in current code: STATUS_MAPS in clients/neovim/lua/badjuju/keymap.lua includes { 'a', 'JJAbandon', ... } (line 27), and the magit-profile branch for status.jujutsu iterates STATUS_MAPS and binds them. The test installs status.jujutsu maps including R/r/n/L/d/q/u/= and a in keymap_spec.lua already verifies this. Docs in docs/src/clients/neovim.md already list a in the status keybinding table. Marking implemented with no code change needed.