Add regex command implementation

tests/fixtures/regex/README.md

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+# Regex Command Fixtures
+
+These fixtures support contract and integration testing for the `renamer regex` command. Each
+subdirectory contains representative filenames used across preview, apply, conflict, and
+validation scenarios.
+
+- `baseline/` — ASCII word + digit combinations (e.g., `alpha-123.log`) used to validate basic
+  capture group substitution.
+- `unicode/` — Multilingual filenames to verify RE2 Unicode handling and ledger persistence.
+- `mixed/` — Build-style artifacts with underscores/dashes for automation-style rename flows.
+- `case-fold/` — Differing only by case to simulate case-insensitive duplicate conflicts.
+
+Tests should copy these directories to temporary working paths before mutation to keep fixtures
+idempotent.