Add configurable sequence numbering command

internal/sequence/plan.go

@@ -0,0 +1,68 @@
+package sequence
+
+// Plan represents the ordered numbering proposal produced during preview.
+type Plan struct {
+	Candidates       []Candidate
+	SkippedConflicts []Conflict
+	Summary          Summary
+	Config           Config
+}
+
+// Candidate describes a single file considered for numbering.
+type Candidate struct {
+	OriginalPath string
+	ProposedPath string
+	Index        int
+	IsDir        bool
+	Status       CandidateStatus
+}
+
+// CandidateStatus indicates how a candidate was handled during preview.
+type CandidateStatus string
+
+const (
+	// CandidatePending means the candidate will be renamed when applied.
+	CandidatePending CandidateStatus = "pending"
+	// CandidateSkipped indicates the candidate was skipped (e.g., conflict).
+	CandidateSkipped CandidateStatus = "skipped"
+	// CandidateUnchanged indicates the candidate already matches the target name.
+	CandidateUnchanged CandidateStatus = "unchanged"
+)
+
+// Conflict captures a skipped item and the reason it could not be renamed.
+type Conflict struct {
+	OriginalPath    string
+	ConflictingPath string
+	Reason          ConflictReason
+}
+
+// ConflictReason enumerates known conflict types.
+type ConflictReason string
+
+const (
+	// ConflictExistingTarget indicates the proposed name collides with an existing file.
+	ConflictExistingTarget ConflictReason = "existing_target"
+	// ConflictInvalidSeparator indicates the proposed separator produced an invalid path.
+	ConflictInvalidSeparator ConflictReason = "invalid_separator"
+	// ConflictWidthOverflow indicates numbering exceeded a fixed width.
+	ConflictWidthOverflow ConflictReason = "width_overflow"
+)
+
+// Summary aggregates totals surfaced during preview.
+type Summary struct {
+	TotalCandidates int
+	RenamedCount    int
+	SkippedCount    int
+	Warnings        []string
+	AppliedWidth    int
+}
+
+// Config snapshots the numbering configuration for preview/apply/ledger.
+type Config struct {
+	Start        int
+	Width        int
+	Placement    Placement
+	Separator    string
+	NumberPrefix string
+	NumberSuffix string
+}