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/plan.md

@@ -0,0 +1,87 @@
+# Implementation Plan: Sequence Numbering Command
+
+**Branch**: `001-sequence-numbering` | **Date**: 2025-11-03 | **Spec**: `specs/001-sequence-numbering/spec.md`
+**Input**: Feature specification from `/specs/001-sequence-numbering/spec.md`
+
+**Note**: This template is filled in by the `/speckit.plan` command. See `.specify/templates/commands/plan.md` for the execution workflow.
+
+## Summary
+
+Deliver a new `renamer sequence` command that appends deterministic sequence numbers to file candidates following the preview-first workflow. The command respects existing scope flags, supports configuration for start value, width, placement, and separator, records batches in the `.renamer` ledger for undo, and skips conflicting filesystem entries while warning the user.
+
+## Technical Context
+
+<!--
+  ACTION REQUIRED: Replace the content in this section with the technical details
+  for the project. The structure here is presented in advisory capacity to guide
+  the iteration process.
+-->
+
+**Language/Version**: Go 1.24  
+**Primary Dependencies**: `spf13/cobra`, `spf13/pflag`, internal traversal/history/output packages  
+**Storage**: Local filesystem + `.renamer` ledger files  
+**Testing**: `go test ./...`, contract + integration suites under `tests/`  
+**Target Platform**: Cross-platform CLI (Linux/macOS/Windows shells)  
+**Project Type**: CLI application (single Go project)  
+**Performance Goals**: Preview + apply 500 files in ≤120s; preview/apply parity ≥95%  
+**Constraints**: Deterministic ordering, atomic ledger writes, skip conflicts while warning  
+**Scale/Scope**: Operates on batches up to hundreds of files per invocation
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+- Preview flow will extend existing preview pipeline to list original → numbered name mappings and highlight skipped conflicts before requiring `--yes`.  
+- Undo strategy leverages ledger entries capturing sequence parameters (start, width, placement) and per-file mappings, ensuring reversal mirrors numbering order.  
+- Sequence rule will be implemented as a composable transformation module declaring inputs (scope candidates + sequence config), validations, and outputs, reusable across preview/apply.  
+- Scope handling continues to consume existing traversal services, honoring `-d`, `-r`, `--extensions`, and leaving directories untouched per clarified requirement while preventing scope escape.  
+- CLI UX will wire flags via Cobra, update help text, and add tests covering flag validation, preview output, warning messaging, and undo flow consistency.
+
+**Post-Design Review:** Research and design artifacts confirm all principles remain satisfied; no constitution waivers required.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/[###-feature]/
+├── plan.md              # This file (/speckit.plan command output)
+├── research.md          # Phase 0 output (/speckit.plan command)
+├── data-model.md        # Phase 1 output (/speckit.plan command)
+├── quickstart.md        # Phase 1 output (/speckit.plan command)
+├── contracts/           # Phase 1 output (/speckit.plan command)
+└── tasks.md             # Phase 2 output (/speckit.tasks command - NOT created by /speckit.plan)
+```
+
+### Source Code (repository root)
+<!--
+  ACTION REQUIRED: Replace the placeholder tree below with the concrete layout
+  for this feature. Delete unused options and expand the chosen structure with
+  real paths (e.g., apps/admin, packages/something). The delivered plan must
+  not include Option labels.
+-->
+
+```text
+cmd/
+├── renamer/               # Cobra CLI entrypoints and command wiring
+internal/
+├── traversal/             # Scope resolution and ordering services
+├── history/               # Ledger and undo utilities
+├── output/                # Preview/summary formatting
+└── sequence/              # [to be added] sequence rule implementation
+tests/
+├── contract/              # CLI contract tests
+├── integration/           # Multi-command flow tests
+└── smoke/                 # Smoke scripts under scripts/
+```
+
+**Structure Decision**: Extend existing single Go CLI project; new logic lives under `internal/sequence`, with command wiring in `cmd/renamer`.
+
+## Complexity Tracking
+
+> **Fill ONLY if Constitution Check has violations that must be justified**
+
+| Violation | Why Needed | Simpler Alternative Rejected Because |
+|-----------|------------|-------------------------------------|
+| [e.g., 4th project] | [current need] | [why 3 projects insufficient] |
+| [e.g., Repository pattern] | [specific problem] | [why direct DB access insufficient] |