feat: add replace subcommand with multi-pattern support

internal/replace/README.md

@@ -0,0 +1,13 @@
+# internal/replace
+
+This package hosts the core building blocks for the `renamer replace` command. The modules are
+organized as follows:
+
+- `request.go` — CLI input parsing, validation, and normalization of pattern/replacement data.
+- `parser.go` — Helpers for token handling (quoting, deduplication, reporting).
+- `traversal.go` — Bridges shared traversal utilities with replace-specific filtering logic.
+- `engine.go` — Applies pattern replacements to candidate names and detects conflicts.
+- `preview.go` / `apply.go` — Orchestrate preview output and apply/ledger integration (added later).
+- `summary.go` — Aggregates match counts and conflict details for previews and ledger entries.
+
+Tests will live alongside the package (unit) and in `tests/contract` + `tests/integration`.