fix: parse boolean flag values case-insensitively

[hiSandog]

Summary

• Parse optional boolean flag values case-insensitively so FALSE and TRUE behave like false and true.
• Consume case-insensitive boolean values in clean_args so they do not leak into command arguments.
• Add unit coverage for parsing and argument cleanup.

Validation

• git diff --check
• Not run: cargo test --manifest-path cli/Cargo.toml flags because cargo/rustc are not installed in this environment.

[vercel]

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