chore(spec): archive completed billing hardening

[matdev83]

Summary

• mark the completed billing hardening spec as completed and not ready for implementation
• add explicit completion status to tasks and archive the spec
• update validation evidence to the merged PR fix(billing): harden post-usage correctness #354 baseline

Verification

• go test ./tools/kiro/speccheck
• make docs-check
• go mod verify
• gofmt and diff checks

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