Add create-issue sub-command to Jira CLI

[Copilot]

This PR adds a new create-issue sub-command to the Jira CLI that allows users to create new Jira issues from the command line.

Usage

jira create-issue <project> <description> [assignee]

Examples

Create an issue without an assignee:

jira create-issue PROJ "Fix the login bug"

Create an issue with an assignee:

jira create-issue PROJ "Implement new feature" john.doe

Implementation Details

The new command:

• Requires a project key (e.g., "PROJ") to specify which project the issue belongs to
• Requires a description that will be used as both the summary and description of the issue
• Accepts an optional assignee username to assign the issue upon creation
• Creates issues with the "Task" issue type by default
• Returns the created issue key (e.g., "PROJ-123") on success

The implementation follows the existing patterns in the codebase:

• Uses the executeCommand() helper for authentication and client setup
• Integrates with the existing command parsing structure
• Provides consistent error handling and output formatting

Changes

• main.go: Added command parsing, validation, and the createIssue() function
• README.md: Updated documentation to include the new command in both the prompt section and usage examples

Testing

✅ Build verification
✅ Argument validation (proper error messages for missing parameters)
✅ CodeQL security scan (0 vulnerabilities found)
✅ No new dependencies added

Note: End-to-end testing requires valid Jira credentials and can be performed using the existing jira configure command to set up authentication.

Original prompt

Add a new sub-command that create a issue. I'll need to be able to provide the project that the issue belongs to and a description of the issue, and an optional assignee.

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[alexec]

@copilot please update PR title to be semantic.