feat(tests): add E2E CLI tests with auto-discovery and coverage

- Black-box E2E tests compiling binary via go build
- Auto-discovery from --help output to catch missing/extra commands
- Golden list (requiredCommands) with bidirectional validation
- Runtime coverage tracking ensuring every command is tested
- Isolated env per test (t.TempDir as HOME)
- JSON envelope schema validation for all demo --json commands
- Schema field checks for accounts and transactions

Author: thedavidweng