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/label_list.go

@@ -0,0 +1,54 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+
+	"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 LabelList struct {
+	c *gitea.Client
+	a *allowlist.Allowlist
+}
+
+func NewLabelList(c *gitea.Client, a *allowlist.Allowlist) *LabelList {
+	return &LabelList{c: c, a: a}
+}
+
+func (t *LabelList) Descriptor() registry.ToolDescriptor {
+	return registry.ToolDescriptor{
+		Name:        "label_list",
+		Description: "List all labels defined on a repo. Returns id, name, and color for each label.",
+		InputSchema: json.RawMessage(`{
+			"type":"object",
+			"properties":{
+				"owner":{"type":"string"},
+				"repo":{"type":"string"}
+			},
+			"required":["owner","repo"]
+		}`),
+	}
+}
+
+type labelListArgs struct {
+	Owner string `json:"owner"`
+	Repo  string `json:"repo"`
+}
+
+func (t *LabelList) Call(ctx context.Context, raw json.RawMessage) (json.RawMessage, error) {
+	var args labelListArgs
+	if err := parseArgs(raw, &args); err != nil {
+		return nil, err
+	}
+	if err := t.a.Check(args.Owner); err != nil {
+		return nil, err
+	}
+	labels, err := t.c.ListLabels(ctx, args.Owner, args.Repo)
+	if err != nil {
+		return nil, err
+	}
+	return textOK(labels)
+}