Fix wallet core configuration, validation, tests, and CI

[ipicoinrepo]

Summary

• document pre-alpha security and release boundaries
• require explicit validated chain configuration
• replace prefix-only address checks with Bech32 decoding and payload validation
• make wallet creation return its instance
• replace placeholder tests with meaningful model and configuration coverage
• add pinned least-privilege CI and remove unsafe automatic Git mutations

Verification

• 7 test files and 11 tests pass on Node.js 24
• high-severity production dependency audit threshold passes
• the remaining low-severity elliptic advisory is inherited and must be removed before a supported release