Detect workspace JSON config

[GreyforgeLabs]

Summary

• Teach workspace show to detect .cortex/config.json when .cortex/config.toml is absent.
• Keep TOML as the preferred workspace config path, while parsing JSON settings when JSON is the discovered file.
• Add a CLI regression test for workspace show --json with only .cortex/config.json.

Fixes PlatformNetwork/bounty-challenge#51499.

Validation

• cargo test -p cortex-cli --test workspace_show --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

[greptile-apps]

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