Refactor ErrorCode to use Result pattern and safe exit code mapping

[z80020100]

Summary

• Remove Success variant from ErrorCode enum — error types should not represent success
• Replace unsafe #[repr(u8)] discriminant access with explicit as_u8() match
• Remove PartialEq/Eq/Termination impls that are no longer needed
• Change main() to return ExitCode and logger.reconfig() to return Result<(), ErrorCode>
• Add strum::EnumCount derive with tests verifying all exit codes are non-zero, unique, and ≤ 125

Closes #37

Test plan

• cargo build passes
• cargo fmt --all -- --check passes
• cargo clippy passes with no warnings
• cargo test passes