feat: add replace subcommand with multi-pattern support

testdata/replace/README.md

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+# Replace Command Fixtures
+
+This directory contains lightweight fixtures for manual and automated testing of the `renamer
+replace` command.
+
+## Structure
+
+```
+replace/
+├── case-sensitivity/
+│   ├── draft.txt
+│   ├── Draft.txt
+│   └── README.md
+├── multi-pattern/
+│   ├── alpha-beta.txt
+│   ├── beta-gamma.log
+│   ├── nested/
+│   │   └── gamma-delta.md
+│   └── README.md
+└── hidden-files/
+    ├── .draft.tmp
+    ├── notes.txt
+    └── README.md
+```
+
+Each subdirectory contains intentionally blank files used to verify preview, apply, and undo
+behaviour. Customize or copy these fixtures as needed for new tests.