Add regex command implementation

internal/regex/summary.go

@@ -0,0 +1,47 @@
+package regex
+
+// Summary describes the outcome of previewing or applying a regex rename request.
+type Summary struct {
+	TotalCandidates int
+	Matched         int
+	Changed         int
+	Skipped         int
+	Conflicts       []Conflict
+	Warnings        []string
+	Entries         []PreviewEntry
+	LedgerMetadata  map[string]any
+}
+
+// ConflictReason enumerates reasons a proposed rename cannot proceed.
+type ConflictReason string
+
+const (
+	ConflictDuplicateTarget ConflictReason = "duplicate_target"
+	ConflictExistingFile    ConflictReason = "existing_file"
+	ConflictExistingDir     ConflictReason = "existing_directory"
+	ConflictInvalidTemplate ConflictReason = "invalid_template"
+)
+
+// Conflict reports a blocked rename candidate to the CLI and callers.
+type Conflict struct {
+	OriginalPath string
+	ProposedPath string
+	Reason       ConflictReason
+}
+
+// EntryStatus captures the preview disposition for a candidate path.
+type EntryStatus string
+
+const (
+	EntryChanged  EntryStatus = "changed"
+	EntryNoChange EntryStatus = "no_change"
+	EntrySkipped  EntryStatus = "skipped"
+)
+
+// PreviewEntry documents a single rename candidate and the proposed output.
+type PreviewEntry struct {
+	OriginalPath string
+	ProposedPath string
+	Status       EntryStatus
+	MatchGroups  []string
+}