Title: Replace unwrap()/expect() calls in core modules with proper error handling

[Mkalbani]

Description

A code audit shows multiple unwrap()/expect() calls in the session, puzzle, and persistence modules that can cause panics on malformed input or I/O failure instead of returning recoverable errors. Replace these with a consistent Result-based error type.

Acceptance Criteria

• A crate-wide GameError enum introduced (using thiserror)
• All unwrap()/expect() in session, puzzle, scoring, and persistence modules replaced with ?-propagated errors, except where panics are truly unreachable and documented as such
• Public API functions return Result<T, GameError> instead of panicking
• CLI layer surfaces errors as user-friendly messages instead of a Rust panic backtrace
• clippy::unwrap_used lint enabled in CI for the affected modules

[jaynomyaro]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.

[Muhamadmust]

Hi maintainer
I have reviewed this issue and would love to work on it, PR 2days

[grantfox-oss]

🦊 GrantFox — @Muhamadmust has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #33)
2. Your PR will be reviewed by the ThinknetCollective maintainers

Good luck! Track your progress on GrantFox.