Add extension normalization command

testdata/extension/README.md

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+# Extension Command Test Data
+
+This directory contains sample files for manual and automated validation of the
+`renamer extension` workflow. To avoid mutating source fixtures directly, copy
+the `sample/` folder to a temporary location before running commands that
+perform real filesystem changes.
+
+Example usage:
+
+```bash
+TMP_DIR=$(mktemp -d)
+cp -R testdata/extension/sample/* "$TMP_DIR/"
+go run ./main.go extension .jpeg .JPG .jpg --path "$TMP_DIR" --dry-run
+```
+
+The sample set includes:
+
+- Mixed-case `.jpeg`/`.JPG` extensions
+- Nested directories (`sample/nested`)
+- An already-normalized `.jpg` file
+- A hidden file to verify `--hidden` opt-in behavior

testdata/extension/sample/.hidden.JPG

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+hidden sample

testdata/extension/sample/image_one.jpeg

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+sample jpeg placeholder

testdata/extension/sample/image_two.JPG

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+second sample

testdata/extension/sample/logo.jpg

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+already normalized

testdata/extension/sample/nested/clip.jpeg

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+nested sample