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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

2 files changed, 33 insertions, 2 deletions

diff --git a/modules/dirvish-config.el b/modules/dirvish-config.el
index e35cc528..6c92db09 100644
--- a/modules/dirvish-config.el
+++ b/modules/dirvish-config.el
@@ -8,6 +8,7 @@
 ;; ediff, playlist creation, path copying, and external file manager integration.
 ;;
 ;; Key Bindings:
+;; - d: Duplicate file at point (adds "-copy" before extension)
 ;; - g: Quick access menu (jump to predefined directories)
 ;; - f: Open system file manager in current directory
 ;; - o/O: Open file with xdg-open/custom command
@@ -145,6 +146,36 @@ Filters for audio files, prompts for the playlist name, and saves the resulting
           (dired-mark 1))
       (forward-line 1))))
 
+;;; ------------------------ Dirvish Duplicate File Copy ------------------------
+
+(defun cj/dirvish-duplicate-file ()
+  "Duplicate the file at point with '-copy' suffix before the extension.
+Examples:
+  report.pdf → report-copy.pdf
+  script.el → script-copy.el
+  README → README-copy"
+  (interactive)
+  (let* ((file (dired-get-filename nil t))
+         (dir (file-name-directory file))
+         (base (file-name-base file))
+         (ext (file-name-extension file t)) ; includes the dot
+         (new-name (concat base "-copy" ext))
+         (new-path (expand-file-name new-name dir)))
+    (unless file
+      (user-error "No file at point"))
+    (when (file-directory-p file)
+      (user-error "Cannot duplicate directories, only files"))
+
+    ;; Check if target already exists
+    (when (file-exists-p new-path)
+      (unless (y-or-n-p (format "File '%s' already exists. Overwrite? " new-name))
+        (user-error "Cancelled")))
+
+    ;; Copy the file
+    (copy-file file new-path t)
+    (revert-buffer)
+    (message "Duplicated: %s → %s" (file-name-nondirectory file) new-name)))
+
 ;;; ----------------------- Dirvish Open File Manager Here ----------------------
 
 (defun cj/dirvish-open-file-manager-here ()
@@ -286,7 +317,7 @@ regardless of what file or subdirectory the point is on."
    ("M-p"     . dirvish-peek-toggle)
    ("M-s"     . dirvish-setup-menu)
    ("TAB"     . dirvish-subtree-toggle)
-   ("d"       . dired-flag-file-deletion)
+   ("d"       . cj/dirvish-duplicate-file)
    ("f"       . cj/dirvish-open-file-manager-here)
    ("g"       . dirvish-quick-access)
    ("o"       . cj/xdg-open)
diff --git a/modules/prog-general.el b/modules/prog-general.el
index d8d9627d..9929228d 100644
--- a/modules/prog-general.el
+++ b/modules/prog-general.el
@@ -251,7 +251,7 @@ If no such file exists there, display a message."
 	  (deadgrep term dir))))
 
 (with-eval-after-load 'dired
-  (keymap-set dired-mode-map "d" #'cj/deadgrep-here))
+  (keymap-set dired-mode-map "D" #'cj/deadgrep-here))
 
 
 ;; ---------------------------------- Snippets ---------------------------------