fix(exec): resolve relative prompt files against --cwd

[694410194]

Summary

• resolve relative --file prompt paths against --cwd when a working directory is provided
• keep absolute prompt file paths unchanged
• add focused path-resolution unit coverage for relative, absolute, and no---cwd cases

Verification

• git diff --check
• static check that build_prompt now reads from resolve_prompt_file_path(file_path, self.cwd.as_deref())
• added focused unit tests for relative path resolution behavior
• rustc --version (not available in this environment, so cargo-based checks could not be run here)

Fixes #53169
/claim #53169

[greptile-apps]

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