Document history default listing

[DanielButler1]

Summary

• document that cortex history lists recent prompts when no subcommand is provided
• add a parser help test for that default behavior
• repair an existing test import so cortex-cli lib tests compile

Related: PlatformNetwork/bounty-challenge#53424

Verification

• rustfmt --edition 2024 --check src/cortex-cli/src/cli/args.rs src/cortex-cli/src/agent_cmd/tests.rs
• git diff --check
• cargo test -p cortex-cli cli::args::tests::test_history_help_documents_default_list_behavior
• cargo check -p cortex-cli

[greptile-apps]

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