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/registry_test.go

@@ -54,5 +54,5 @@ func TestEveryRegisteredToolIsDispatchable(t *testing.T) {
 // Lock the tool count so an accidental drop of a registration in RegisterAll
 // (the single source main.go and this test share) fails loudly.
 func TestRegisteredToolCount(t *testing.T) {
-	assert.Len(t, buildRegistry().Tools(), 39)
+	assert.Len(t, buildRegistry().Tools(), 41)
 }