fix: default manual payments to success in MCP

[sudip-zenskar]

Summary

• default MCP createPayment status metadata to success
• restrict createPayment status choices to success so manual/offline recorded payments are realized
• update README payment wording to match the MCP tool contract

Validation

• node -e JSON.parse(...) for src/mcp-config.json: passed
• pnpm run test: passed, 23/23
• git diff --check: passed
• pnpm run build: blocked by existing tsdown dependency issue: Failed to import module "unrun"

[chirag-zenskar]

Approved. The tool metadata now scopes createPayment to the frontend Add Payment-style successful manual/tax-withheld flow, and timestamp control has been removed so the backend assigns the recorded time.