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_test.go

@@ -0,0 +1,107 @@
+package gitea_test
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"git.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestListLabels(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodGet, r.Method)
+		assert.Equal(t, "/api/v1/repos/o/r/labels", r.URL.Path)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[
+			{"id":1,"name":"bug","color":"ee0701"},
+			{"id":2,"name":"enhancement","color":"84b6eb"}
+		]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	labels, err := c.ListLabels(context.Background(), "o", "r")
+	require.NoError(t, err)
+	require.Len(t, labels, 2)
+	assert.Equal(t, int64(1), labels[0].ID)
+	assert.Equal(t, "bug", labels[0].Name)
+	assert.Equal(t, "ee0701", labels[0].Color)
+	assert.Equal(t, "enhancement", labels[1].Name)
+}
+
+func TestListLabels_Empty(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	labels, err := c.ListLabels(context.Background(), "o", "r")
+	require.NoError(t, err)
+	assert.Empty(t, labels)
+}
+
+func TestListLabels_NotFound(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"message":"repo not found"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	_, err := c.ListLabels(context.Background(), "o", "r")
+	require.Error(t, err)
+	assert.ErrorIs(t, err, gitea.ErrNotFound)
+}
+
+func TestAddIssueLabels(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPost, r.Method)
+		assert.Equal(t, "/api/v1/repos/o/r/issues/42/labels", r.URL.Path)
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`[
+			{"id":1,"name":"bug","color":"ee0701"},
+			{"id":3,"name":"priority","color":"00ff00"}
+		]`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	labels, err := c.AddIssueLabels(context.Background(), "o", "r", 42, []int64{3})
+	require.NoError(t, err)
+
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	ids, ok := payload["labels"].([]any)
+	require.True(t, ok)
+	require.Len(t, ids, 1)
+	assert.Equal(t, float64(3), ids[0])
+
+	require.Len(t, labels, 2)
+	assert.Equal(t, "bug", labels[0].Name)
+	assert.Equal(t, "priority", labels[1].Name)
+}
+
+func TestAddIssueLabels_NotFound(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"message":"issue not found"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	_, err := c.AddIssueLabels(context.Background(), "o", "r", 999, []int64{1})
+	require.Error(t, err)
+	assert.ErrorIs(t, err, gitea.ErrNotFound)
+}