✨ feat(mq-lang,mq-run): add --allow-read runtime flag for filesystem reads

[harehare]

Summary

read_file/read_file_bytes were only gated by the compile-time file-io Cargo feature, unlike http()/write_file which also require an explicit runtime opt-in (--allow-net/--allow-write). This let a third-party module fetched via HTTP import silently read local files. Add a process-wide READ_ALLOWED flag (capability::set_allow_read/is_read_allowed) and the --allow-read CLI flag to close that gap, matching the existing pattern.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation
• ⚡ Performance
• ✅ Test
• 📦 Build / dependencies
• 👷 CI

Checklist

• I ran cargo fmt and cargo clippy and addressed any warnings
• I ran just test-all and all tests pass
• I added or updated tests covering this change
• I updated relevant documentation (/docs, crate README.md) if needed
• I added a changelog entry if this is a user-facing change

Additional Context

[codspeed-hq]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing feat/allow-read-flag (28708ac) with main (0e6922d)}