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/labels.go

@@ -0,0 +1,48 @@
+package gitea
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+// ListLabels fetches all labels defined on a repo.
+func (c *Client) ListLabels(ctx context.Context, owner, repo string) ([]Label, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/labels", owner, repo)
+	body, status, err := c.GetJSON(ctx, p)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var labels []Label
+	if err := json.Unmarshal(body, &labels); err != nil {
+		return nil, err
+	}
+	return labels, nil
+}
+
+// AddIssueLabels adds labelIDs to an issue or pull request (PRs share index
+// space with issues, per Gitea). This is additive per Gitea's own POST
+// semantics — existing labels are left in place, no replace/delete needed.
+// Returns the issue's full label set after the add.
+func (c *Client) AddIssueLabels(ctx context.Context, owner, repo string, number int, labelIDs []int64) ([]Label, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/issues/%d/labels", owner, repo, number)
+	payload, err := json.Marshal(map[string][]int64{"labels": labelIDs})
+	if err != nil {
+		return nil, err
+	}
+	body, status, err := c.PostJSON(ctx, p, payload)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var labels []Label
+	if err := json.Unmarshal(body, &labels); err != nil {
+		return nil, err
+	}
+	return labels, nil
+}