Initialize spec-driven development (GitHub Spec Kit) (#5235)
## Summary - Add GitHub Spec Kit scaffold (`.specify/` directory, `.claude/commands/` slash commands) - Write project constitution capturing wallet's principles: maintenance-first stability, era-aware design, type safety, formal specification, reproducible builds, comprehensive testing, and code quality gates - Add `.gitignore` exception for `.specify/memory/` (blocked by existing bare `memory` pattern) ## Test plan - [ ] Verify `.specify/` directory structure is complete - [ ] Verify constitution reflects project conventions - [ ] Verify slash commands are available in Claude Code sessions