fix(cli): require workspace config to be a file

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53136.

workspace show now treats .cortex/config.toml as present only when it is a regular file. A directory at that path is reported as not configured and settings loading returns None, avoiding the previous has_cortex_config: true / settings: null false positive.

Validation:

• cargo +1.90.0 fmt --package cortex-cli --check
• isolated rustc +1.90.0 --test harness covering config file vs config directory detection
• git diff --check

Note: full cargo +1.90.0 test -p cortex-cli ... is blocked on this Windows machine by the existing workspace aws-lc-sys MSVC build failure before cortex-cli tests run.

[greptile-apps]

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