fix(cli): require cortex workspace marker directory

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53138.

find_workspace_root() now only treats .cortex as a workspace marker when it is a directory. A plain file named .cortex in a nested directory is ignored, so root detection continues upward to the actual workspace marker. The .git fallback continues to use exists() so Git worktree .git files still work.

Validation:

• cargo +1.90.0 fmt --package cortex-cli --check
• isolated rustc +1.90.0 --test harness covering .cortex file ignored and .git file fallback retained
• git diff --check

Note: cargo +1.90.0 test -p cortex-cli test_find_workspace_root -- --nocapture 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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