Summary

• Migrate base64, date, and cut builtins from manual while i < ctx.args.len() loops to ArgParser
• First batch of the incremental migration described in Migrate 27 builtins from manual arg parsing to ArgParser #880
• Net: -5 lines (76 added, 81 removed) — cleaner, less error-prone arg handling

Test plan

• All 1929 existing bashkit tests pass
• cargo clippy -- -D warnings clean
• cargo fmt --check clean
• base64 encode/decode/wrap/roundtrip tests pass
• date and cut spec_case tests pass

Ref #880