Add workspace unset command

[GreyforgeLabs]

Summary

• Add cortex workspace unset <key> and wire it into the workspace subcommand parser.
• Reuse the workspace set key mapping so aliases like model, sandbox_mode, and dotted keys remove the matching TOML entry.
• Add binary CLI coverage for removing mapped and top-level keys plus missing-key errors.

Fixes PlatformNetwork/bounty-challenge#52468.

Validation

• cargo test -p cortex-cli --test workspace_unset --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check
• git diff --cached --check

[greptile-apps]

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