Architecture Context

Replace the independent file-write loop in cmd/waza/cmd_new.go:351-363 with the shared FileWriter from #48. This ensures waza new has the same always-visible inventory behavior as waza init. The existing SKILL.md malformed-detection logic (detectExistingSkillMD) should remain - it runs before the FileWriter and decides whether to include SKILL.md in the write list.

Architecture doc: docs/init-config-consolidation/architecture.md
Depends on: #48

Acceptance Criteria

• cmd/waza/cmd_new.go uses FileWriter from internal/scaffold/ instead of its inline write loop
• Running waza new my-skill shows created marker for new files and exists marker for existing files
• Malformed SKILL.md detection still works - replaces invalid SKILL.md with defaults
• Inventory is always visible (not gated behind --verbose)
• Existing TestNewCommand_NoOverwriteSafety and TestNewCommand_IdempotentRunTwice still pass
• go test ./... passes; golangci-lint run passes