Add regex command implementation

testdata/regex/README.md

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+# Regex Command Scenario Fixtures
+
+This directory provides ready-to-run datasets for validating the `renamer regex`
+command against realistic workflows. Copy a scenario to a temporary directory
+before mutating files so the repository remains clean:
+
+```bash
+TMP_DIR=$(mktemp -d)
+cp -R testdata/regex/capture-groups/* "$TMP_DIR/"
+go run ./main.go regex '^(\w+)-(\d+)$' '@2_@1' --dry-run --path "$TMP_DIR"
+```
+
+## Structure
+
+```
+regex/
+├── capture-groups/
+│   ├── alpha-123.log
+│   ├── beta-456.log
+│   ├── gamma-789.log
+│   └── notes.txt
+├── automation/
+│   ├── build_101_release.tar.gz
+│   ├── build_102_hotfix.tar.gz
+│   ├── build_103_varchive/
+│   │   └── placeholder.txt
+│   └── feature-demo_2025-10-01.txt
+└── validation/
+    ├── duplicate-a-01.txt
+    ├── duplicate-b-01.txt
+    └── group-miss.txt
+```
+
+### Scenario Highlights
+
+- **capture-groups** – Mirrors the quickstart preview example. Run with
+  `renamer regex '^(\w+)-(\d+)$' '@2_@1' --dry-run` to verify captured groups
+  swap safely while non-matching files remain untouched.
+- **automation** – Supports end-to-end `--yes` applies and undo. Use
+  `renamer regex '^(feature)-(.*)$' '@2-@1' --yes` to exercise ledger writes
+  and `renamer regex '^(build)_(\d+)_v(.*)$' 'release-@2-@1-v@3'` to combine
+  extension filtering with directory handling.
+- **validation** – Surfaces error cases. Applying
+  `renamer regex '^(duplicate)-(.*)-(\d+)$' '@1-@3' --yes` should report a
+  duplicate-target conflict, while referencing `@2` with
+  `renamer regex '^(.+)$' '@2' --dry-run` raises an undefined group error.
+
+Extend these fixtures as new edge cases or regression scenarios arise.