feat: add replace subcommand with multi-pattern support

tests/fixtures/replace-samples/README.md

@@ -0,0 +1,9 @@
+# Replace Command Fixtures
+
+Use this directory to store sample file trees referenced by replace command tests. Keep fixtures
+minimal and platform-neutral (ASCII names, small files). Create subdirectories per scenario, e.g.:
+
+- `basic/` — Simple files demonstrating multi-pattern replacements.
+- `conflicts/` — Cases that intentionally trigger name collisions for validation.
+
+File contents are typically irrelevant; create empty files unless a test requires data.