feat: add issue create command

[hex2dec]

Summary

• add an issue create command to the CLI
• support explicit --repo input and local repository inference
• support issue body input from --body, --body-file, and stdin via --body-file -
• keep text and JSON output stable, and align auth / repo-context / remote error handling with existing CLI behavior
• add integration coverage for success and failure paths

Validation

• cargo fmt -- --check
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings

Sample output

Text output:
Created issue I125
repository: octo/demo
title: Use explicit repo
state: open
author: bob
source: explicit
url: https://gitee.com/octo/demo/issues/I125

JSON output:
{"source":"local","owner":"octo","name":"demo","number":"I124","title":"Add issue create","state":"open","author":"alice","body":"Creates an issue from the CLI","comments":0,"html_url":"https://gitee.com/octo/demo/issues/I124","created_at":"2026-03-20T14:00:00Z","updated_at":"2026-03-20T14:00:00Z"}

Closes #43