Add invoice approval workflow test documentation

[nonggde]

Closes #608

Summary

• repair the folder-local Invoice Approval Workflow README and specs placeholders
• add a contributor-facing test plan and review notes
• add synthetic invoice approval fixture cases covering ready-for-approval, needs-review, blocked, and rejected states
• add a zero-dependency Node documentation/fixture contract test

Scope

• only touches tools/v2/team/invoice-approval-workflow/
• no app shell, routing, auth, wallet, Stellar, database, inbox, mail rendering, payment execution, accounting export, or design-system integration
• no live network calls, secrets, production data, persistence, or side effects

Validation

• node --test tools/v2/team/invoice-approval-workflow/tests/invoice-approval-docs.test.mjs
• npx --yes prettier --check on the changed Invoice Approval Workflow files
• git diff --check
• ASCII scan over tools/v2/team/invoice-approval-workflow