fix: create parent directories and improve error messages when saving diagrams

[Copilot]

Addresses feedback from #60: writeFileSync would throw cryptic errors when --output pointed to non-existent directories.

Changes

• Auto-create parent directories: Use mkdirSync(parentDir, {recursive: true}) before writing diagram files
• Actionable error messages: Catch and translate filesystem errors into clear guidance:
  • EACCES → "Permission denied writing to {path}. Check file permissions."
  • ENOSPC → "No space left on device when writing to {path}."
  • EROFS → "Cannot write to {path}: read-only file system."
  • Fallback includes original error message
• Test coverage: Added integration test verifying diagram saves to nested non-existent paths like /tmp/xyz/nested/path/diagram.png

Example

# Previously failed with cryptic ENOENT
c8ctl get pi --key 123 --diagram --output ./reports/2024/diagram.png

# Now auto-creates ./reports/2024/ and saves diagram

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