Add parent-issue support to issue update command

[adoval4]

Summary

This PR adds support for setting and removing parent-child relationships between Linear issues using the linctl issue update command.

Features Added

• New --parent-issue flag for the issue update command
• Support for setting parent issues using issue identifiers (e.g., LIN-123)
• Support for removing parent assignments with unassigned, none, or empty string
• Validation to prevent self-reference (issue cannot be its own parent)
• Validation to ensure parent issue exists before assignment
• Enhanced GraphQL query to include parent information in responses

Usage Examples

# Set a parent issue
linctl issue update LIN-123 --parent-issue LIN-456

# Remove parent assignment
linctl issue update LIN-123 --parent-issue unassigned

# Combine with other updates
linctl issue update LIN-123 --parent-issue LIN-456 --title "New title" --priority 2

Implementation Details

• Uses Linear's parentId field in IssueUpdateInput GraphQL mutation
• Converts issue identifiers to UUIDs for API compatibility
• Includes comprehensive error handling and validation
• Maintains backward compatibility with existing functionality
• All existing tests continue to pass

Testing

• Manual testing with real Linear issues confirms functionality
• Validation edge cases tested (self-reference, non-existent issues)
• All smoke tests pass with no regressions
• Combined field updates work correctly

This implementation follows the existing codebase patterns and maintains the same error handling and output formatting standards used throughout the CLI tool.

[adoval4]

@dorkitude what do you think about this?

[memyselfandm]

Closing in favor of PR #24 which has been merged.

Thank you @adoval4 for your contribution! Your comprehensive README documentation has been ported to PR #24 (adapted for the --parent flag naming).

Key differences in the merged implementation:

• Flag name: --parent (shorter, consistent with other flags like --assignee)
• Removal keyword: none instead of unassigned (more semantically appropriate for parent relationships)

Your documentation examples were invaluable - thanks for the contribution!