feat: add issue list --label flag for filtering by label

[q-soriarty]

Problem

`copia-cli issue list --label bug` returns `Error: unknown flag: --label`.

The `gh` CLI supports `--label` on `issue list` to filter issues by label.

Expected behavior

```
$ copia-cli issue list --label bug
#42 Fix sensor mapping open bug
```

Implementation

Add `--label` string slice flag. Pass to Gitea API as `?labels=` query parameter.

Discovered by

gh CLI behavior comparison test.

[github-actions]

Closed by #121 (merged into develop).