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/tools/issue_label.go

@@ -0,0 +1,87 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"git.d-ma.be/mathias/gitea-mcp/internal/allowlist"
+	"git.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"git.d-ma.be/mathias/gitea-mcp/internal/registry"
+)
+
+type IssueLabel struct {
+	c *gitea.Client
+	a *allowlist.Allowlist
+}
+
+func NewIssueLabel(c *gitea.Client, a *allowlist.Allowlist) *IssueLabel {
+	return &IssueLabel{c: c, a: a}
+}
+
+func (t *IssueLabel) Descriptor() registry.ToolDescriptor {
+	return registry.ToolDescriptor{
+		Name:        "issue_label",
+		Description: "Add labels to an issue or pull request. Resolves label names to IDs via the repo's label list. Additive — existing labels are left in place.",
+		InputSchema: json.RawMessage(`{
+			"type":"object",
+			"properties":{
+				"owner":{"type":"string"},
+				"repo":{"type":"string"},
+				"number":{"type":"integer","minimum":1},
+				"labels":{"type":"array","items":{"type":"string"}},
+				"label_ids":{"type":"array","items":{"type":"integer"}}
+			},
+			"required":["owner","repo","number","labels"]
+		}`),
+	}
+}
+
+type issueLabelArgs struct {
+	Owner    string   `json:"owner"`
+	Repo     string   `json:"repo"`
+	Number   int      `json:"number"`
+	Labels   []string `json:"labels,omitempty"`
+	LabelIDs []int64  `json:"label_ids,omitempty"`
+}
+
+func (t *IssueLabel) Call(ctx context.Context, raw json.RawMessage) (json.RawMessage, error) {
+	var args issueLabelArgs
+	if err := parseArgs(raw, &args); err != nil {
+		return nil, err
+	}
+	if err := t.a.Check(args.Owner); err != nil {
+		return nil, err
+	}
+	if args.Number < 1 {
+		return nil, fmt.Errorf("number must be >= 1: %w", gitea.ErrValidation)
+	}
+	if len(args.Labels) == 0 && len(args.LabelIDs) == 0 {
+		return nil, fmt.Errorf("labels is required: %w", gitea.ErrValidation)
+	}
+
+	ids := append([]int64{}, args.LabelIDs...)
+	if len(args.Labels) > 0 {
+		existing, err := t.c.ListLabels(ctx, args.Owner, args.Repo)
+		if err != nil {
+			return nil, err
+		}
+		byName := make(map[string]int64, len(existing))
+		for _, l := range existing {
+			byName[l.Name] = l.ID
+		}
+		for _, name := range args.Labels {
+			id, ok := byName[name]
+			if !ok {
+				return nil, fmt.Errorf("label %q not found in %s/%s: %w", name, args.Owner, args.Repo, gitea.ErrValidation)
+			}
+			ids = append(ids, id)
+		}
+	}
+
+	labels, err := t.c.AddIssueLabels(ctx, args.Owner, args.Repo, args.Number, ids)
+	if err != nil {
+		return nil, err
+	}
+	return textOK(labels)
+}