feat(labels): add label_list + issue_label tools (#52)

Closes #52 — unblocks parallax#3 dispatch labeling, which needs to both
discover a repo's label set and attach labels to an issue by name (the
CAD pipeline doesn't track Gitea's internal numeric label IDs).

- gitea.Client: ListLabels, AddIssueLabels (additive POST, matches
  Gitea's own semantics — no delete-then-post needed); extend the
  existing Label struct with Color for label_list's output.
- tools.LabelList (read-only, allowlisted): lists a repo's labels.
- tools.IssueLabel (allowlisted): resolves label names to IDs via
  ListLabels, so callers pass names (the primary interface) instead of
  hunting for numeric IDs; also accepts label_ids for callers that
  already have them. An unknown name fails closed, naming exactly which
  label wasn't found.
- Bump TestRegisteredToolCount 39 -> 41 in the same commit (this
  project was bitten today by a locked count going stale silently).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/gitea/issues.go

@@ -22,8 +22,9 @@ type Issue struct {
 }
 
 type Label struct {
-	ID   int64  `json:"id"`
-	Name string `json:"name"`
+	ID    int64  `json:"id"`
+	Name  string `json:"name"`
+	Color string `json:"color,omitempty"`
 }
 
 type User struct {