feat(dirvish): add file duplication with 'd' key, move deadgrep to 'D'

This commit adds a convenient file duplication function to dirvish and
reorganizes keybindings to avoid conflicts.

## Changes

**1. New function: cj/dirvish-duplicate-file (dirvish-config.el:150)**

Duplicates the file at point with "-copy" suffix before the extension:
- report.pdf → report-copy.pdf
- script.el → script-copy.el
- README → README-copy

Features:
- Prevents duplicating directories (files only)
- Checks if target exists and prompts to overwrite
- Refreshes buffer automatically after copying
- Shows clear message with old and new names

**2. Keybinding changes**

dirvish-config.el:
- Bound 'd' to cj/dirvish-duplicate-file (was dired-flag-file-deletion)
- Updated Commentary section to document new binding

prog-general.el:
- Moved cj/deadgrep-here from 'd' to 'D' (capital D)
- More mnemonic: D for Deadgrep
- Avoids conflict with new duplicate function

## Rationale

The 'd' key is prime real estate in file managers, and duplicating files
is a very common operation. The standard dired-flag-file-deletion is still
available via 'x' or the mark-and-delete workflow.

Deadgrep on 'D' is more discoverable (capital D for Deadgrep) and less
likely to be pressed accidentally.

## Usage

In dirvish, navigate to a file and press:
- 'd' - Duplicate file with "-copy" suffix
- 'D' - Search with deadgrep in current directory

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