feat: add recursive flag to interpolate command

[amountainram]

Description:

This PR adds a --recursive (-r) flag to the interpolate command, allowing it to recursively process files in subdirectories.

Changes

• Added --recursive / -r boolean flag to the interpolate command
• When enabled, subdirectories are walked recursively instead of being skipped
• Output preserves the relative directory structure from input paths
• Added relativeOutputName helper to compute relative output paths
• Added test case and expected test data for recursive interpolation

Notes

• Without --recursive, behavior is unchanged (subdirectories are skipped)
• Output directories are created automatically via MkdirAll