feat(explorer): Add auto_open_on_cursor and auto_open_debounce_ms options

[craftzdog]

Hi! I love this plugin for my agentic workflow. I wanted to add a feature that allows you to automatically open the diff for the file under the cursor so that I can review files more quickly.

Summary

Adds an opt-in mode that auto-opens the diff for the file under the cursor as you move through the explorer with j/k, so you can browse changes without pressing Enter on each file.

Changes

• auto_open_on_cursor (default false) — when enabled, moving the cursor in the explorer opens the diff for the node beneath it.
• auto_open_debounce_ms (default 80) — debounce window so fast j/k traversal doesn't spam diff loads.

The CursorMoved handler is debounced and skips group/directory nodes and the already-open file. Its timer is cleaned up on BufWipeout.

Config

explorer = {
  auto_open_on_cursor = false,  -- auto-open diff for file under cursor while moving
  auto_open_debounce_ms = 80,   -- debounce window (ms) for the above
}

Test plan

• Unit tests in tests/ui/explorer/explorer_spec.lua cover the debounce behavior, node-type skipping, and duplicate-file guards.