fix: escape rewrite replacement, diagnostic no-op commit, doc + askpass hardening

- Use function replacer in rewriteBody to prevent $& / $$ / $` / $' pattern
  corruption when alt text or siteUrl contains dollar-sign sequences
- Detect empty staged index after `git add -A` and throw a clear
  "No changes to publish" error instead of a cryptic git failure
- Correct README flat-slug description: single image → <slug>.<ext>,
  multiple → <slug>-1.<ext>, <slug>-2.<ext> (original filename discarded)
- Harden askpass dir resolution: show a Notice and return early if
  getFullPath is absent (desktop-only guard), rather than passing a bad path

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

README.md

@@ -36,7 +36,7 @@ An Obsidian plugin that publishes the active note as a Jekyll blog post via git.
 
 ## Image strategies
 
-**flat-slug** — all images for the post are copied flat into `<images-dir>/` and renamed to `<slug>-<original-name>`. Keeps the images directory shallow; suitable for sites with few images per post.
+**flat-slug** — all images for the post are copied flat into `<images-dir>/`. A single image is renamed to `<slug>.<ext>`; multiple images are renamed to `<slug>-1.<ext>`, `<slug>-2.<ext>`, … (the original Obsidian filename is discarded). Keeps the images directory shallow; suitable for sites with few images per post.
 
 **per-post-folder** — images are copied into `<images-dir>/<slug>/` preserving their original filenames. Keeps each post's images grouped together; suitable for posts with many images.