Add extension normalization command

specs/004-extension-rename/quickstart.md

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+# Quickstart – Extension Normalization Command
+
+1. **Preview extension changes.**
+   ```bash
+   renamer extension .jpeg .JPG .jpg --dry-run
+   ```
+   - Shows original → proposed paths.
+   - Highlights conflicts or “no change” rows for files already ending in `.jpg`.
+
+2. **Include nested directories or hidden assets when needed.**
+   ```bash
+   renamer extension .yaml .yml .yml --recursive --hidden --dry-run
+   ```
+   - `--recursive` traverses subdirectories.
+   - `--hidden` opt-in keeps hidden files in scope.
+
+3. **Apply changes after confirming preview.**
+   ```bash
+   renamer extension .jpeg .JPG .jpg --yes
+   ```
+   - `--yes` auto-confirms preview results.
+   - Command exits `0` even if no files matched (prints “no candidates found”).
+
+4. **Undo the most recent batch if needed.**
+   ```bash
+   renamer undo
+   ```
+   - Restores original extensions using `.renamer` ledger entry.