feat(sequence-numbering): Implement sequence numbering command with comprehensive specifications

- Removed obsolete tasks.md from 001-sequence-numbering.
- Added detailed requirements checklist for sequence numbering command.
- Created OpenAPI contract for sequence command, defining preview and apply endpoints.
- Developed data model for sequence numbering, outlining key entities and their relationships.
- Drafted implementation plan for sequence numbering command, including project structure and complexity tracking.
- Compiled quickstart guide for sequence numbering command, detailing prerequisites and usage examples.
- Documented research findings related to command validation, ordering, and conflict handling.
- Established feature specification for sequence numbering command, including user scenarios and acceptance criteria.
- Created tasks.md for 007-sequence-numbering, outlining phases and implementation strategy.

specs/007-sequence-numbering/data-model.md

@@ -0,0 +1,68 @@
+# Data Model: Sequence Numbering Command
+
+## SequenceRequest
+- **Fields**
+  - `Path` (string): Root path for traversal (defaults to current directory); must exist and be accessible.
+  - `Recursive` (bool): Includes subdirectories when true.
+  - `IncludeDirs` (bool): Includes directories in traversal results without numbering.
+  - `Hidden` (bool): Includes hidden files when true.
+  - `Extensions` ([]string): Optional `.`-prefixed extension filter; deduplicated and validated.
+  - `DryRun` (bool): Indicates preview-only execution.
+  - `Yes` (bool): Confirmation flag for apply mode.
+  - `Start` (int): First sequence value; must be ≥1.
+  - `Width` (int): Minimum digits for zero padding; must be ≥1 when provided.
+  - `Placement` (enum: `prefix` | `suffix`): Determines where the sequence number is inserted; default `suffix`.
+  - `Separator` (string): Separator between number and filename; defaults to `_`; must comply with filesystem rules (no path separators, non-empty).
+- **Relationships**: Consumed by traversal service to produce candidates and by sequence rule to generate `SequencePlan`.
+- **Validations**: Numeric fields validated before preview; separator sanitized; conflicting flags (dry-run vs yes) rejected.
+
+## SequencePlan
+- **Fields**
+  - `Candidates` ([]SequenceCandidate): Ordered list of files considered for numbering.
+  - `SkippedConflicts` ([]SequenceConflict): Files skipped due to target path collisions.
+  - `Summary` (SequenceSummary): Counts for total candidates, renamed files, and skipped items.
+  - `Config` (SequenceConfig): Snapshot of sequence settings (start, width, placement, separator).
+- **Relationships**: Passed to output package for preview rendering and to history package for ledger persistence.
+- **Validations**: Candidate ordering must match traversal ordering; conflicts identified before apply.
+
+### SequenceCandidate
+- **Fields**
+  - `OriginalPath` (string)
+  - `ProposedPath` (string)
+  - `Index` (int): Zero-based position used to derive padded number.
+- **Constraints**: Proposed path must differ from original to be considered a rename; duplicates flagged as conflicts.
+
+### SequenceConflict
+- **Fields**
+  - `OriginalPath` (string)
+  - `ConflictingPath` (string)
+  - `Reason` (string enum: `existing_target`, `invalid_separator`, `width_overflow`)
+
+### SequenceSummary
+- **Fields**
+  - `TotalCandidates` (int)
+  - `RenamedCount` (int)
+  - `SkippedCount` (int)
+  - `Warnings` ([]string)
+
+## SequenceLedgerEntry
+- **Fields**
+  - `Timestamp` (time.Time)
+  - `Rule` (string): Fixed value `sequence`.
+  - `Config` (SequenceConfig): Stored to support undo.
+  - `Operations` ([]SequenceOperation): Each captures `From` and `To` paths actually renamed.
+- **Relationships**: Append-only entry written by history package; consumed by undo command.
+- **Validations**: Only include successful renames (skipped conflicts omitted). Undo must verify files still exist before attempting reversal.
+
+### SequenceOperation
+- **Fields**
+  - `From` (string)
+  - `To` (string)
+
+## SequenceConfig
+- **Fields**
+  - `Start` (int)
+  - `Width` (int)
+  - `Placement` (string)
+  - `Separator` (string)
+- **Usage**: Embedded in plan summaries, ledger entries, and undo operations to ensure consistent behavior across preview and apply.