fix: report unreadable instruction files

[CoderDeltaLAN]

Summary

• Reports supported instruction files that cannot be analyzed as UTF-8 as AIRK-SYS001 findings.
• Prevents discovered instruction files from being silently skipped by governance checks.
• Documents AIRK-SYS001 in README, docs/RULES.md, and CHANGELOG.

Verification

• Focused governance and CLI tests pass.
• Full local check passes from a temporary development virtual environment with .[dev].
• Branch diff scope and secret-pattern scan were checked before push.