fix: throw clear error when cwd directory does not exist

[poetryofcode]

fix: throw clear error when cwd directory does not exist

Summary

• Adds early validation for the cwd option before executing commands
• Throws a clear Fail error if the directory doesn't exist or isn't a
  directory
• Improves DX by providing actionable error messages instead of cryptic
  spawn errors

Before: Users got cryptic errors like "Top-level await promise never
resolved"
After: Users get clear messages like The cwd directory does not exist: /path/to/dir

Test plan

• Added test for non-existent cwd directory
• Added test for cwd pointing to a file instead of directory
• Verified existing functionality still works
• Updated size limits to accommodate the ~320 byte increase

Fixes #1386

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