Resolve exec prompt files relative to --cwd

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53169.

Summary

• Resolve relative cortex exec --file paths against --cwd when a working directory is provided.
• Keep absolute --file paths unchanged and preserve existing relative behavior when --cwd is omitted.
• Add regression coverage for each path resolution case.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check
• Isolated rustc +1.90.0 --edition=2024 --test harness covering relative-with-cwd, relative-without-cwd, and absolute path resolution

Note: targeted cargo test -p cortex-cli resolves_relative_prompt_file_against_cwd -- --nocapture is still blocked before running tests on this Windows/MSVC environment by the existing aws-lc-sys C build failure.

[greptile-apps]

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