Clarify lock command usage

[GreyforgeLabs]

Summary

• add explicit usage forms for cortex lock so the default session-id mode and subcommand mode are not shown as [SESSION_ID] [COMMAND]
• keep the existing shorthand cortex lock [SESSION_ID] behavior intact

Fixes PlatformNetwork/bounty-challenge#53143

Tests

• cargo fmt --package cortex-cli --check
• git diff --check
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo run -p cortex-cli --no-default-features --features cortex-tui --bin Cortex -- lock --help

[greptile-apps]

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