feat: add pr close --comment and --delete-branch flags

[q-soriarty]

Problem

`gh pr close` supports `--comment` and `--delete-branch` flags. `copia-cli pr close` has neither.

gh reference

```
gh pr close 7 --comment "Closing in favor of #8" --delete-branch
```

Scope

• `--comment string` — leave a closing comment (like `issue close --comment`)
• `--delete-branch` — delete local and remote branch after close

Discovered by

gh CLI behavior comparison test.

[github-actions]

Closed by #122 (merged into develop).