hive-forge: add repo-labels verb to list a repo's full label set

`labels <number>` only lists an issue/PR's labels; there was no way to
list the project-wide label set, which triage/labelling needs to discover
valid names. Add `repo-labels [pattern]` hitting GET /repos/{owner}/{repo}/labels
(paginated), with optional name-substring filter. Default prints one label
per line (name + tab-separated description when set); --json emits the full
label objects.

hive-forge/src/verbs/mod.rs

@@ -31,6 +31,7 @@ pub mod pr_reviews;
 pub mod pr_status;
 pub mod repo_add_collaborator;
 pub mod repo_create;
+pub mod repo_labels;
 pub mod subscription;
 pub mod timeline;
 pub mod tree_sha;