fix(dirvish): move deadgrep from D to G, preserve dired-do-delete on D

The previous commit incorrectly moved deadgrep to 'D', which conflicts
with the important dired-do-delete command (immediate delete of marked
files). This commit moves deadgrep to 'G' instead.

## Changes

**prog-general.el:**
- Move cj/deadgrep-here from 'D' to 'G' (was dired-do-chgrp, rarely used)

**dirvish-config.el:**
- Updated Commentary to document both 'D' and 'G' keybindings
- 'D' remains dired-do-delete (standard dired immediate delete)
- 'G' is now deadgrep search

## Final Keybinding Map

| Key | Binding                       | Purpose                        |
|-----|-------------------------------|--------------------------------|
| d   | cj/dirvish-duplicate-file     | Duplicate with "-copy" suffix  |
| D   | dired-do-delete               | Delete marked files (standard) |
| g   | dirvish-quick-access          | Quick directory menu           |
| G   | cj/deadgrep-here              | Search with deadgrep           |

Capital G is mnemonic for "Grep" and doesn't conflict with important
dired commands. dired-do-chgrp is rarely used and not a loss.

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