fix(issue_label): schema wrongly required labels, blocking label_ids-only callers (#52 review finding)

Independent adversarial review of #52 (v0.8.0) caught a schema/implementation
mismatch: the advertised InputSchema marked "labels" as required, but Call
already treated labels/label_ids as either-or. An MCP client that validates
arguments against the advertised schema before dispatch would reject a
label_ids-only call as invalid even though the code was written to serve it —
and that path had zero test coverage either way.

Dropped "labels" from the required array (owner/repo/number remain required);
runtime validation already correctly requires at least one of labels/label_ids.
Added TestIssueLabelAppliesByIDOnly (asserts ListLabels is never called when
IDs are already known) and a schema-lock test for the fixed contract.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/issue_label.go

@@ -29,10 +29,10 @@ func (t *IssueLabel) Descriptor() registry.ToolDescriptor {
 				"owner":{"type":"string"},
 				"repo":{"type":"string"},
 				"number":{"type":"integer","minimum":1},
-				"labels":{"type":"array","items":{"type":"string"}},
-				"label_ids":{"type":"array","items":{"type":"integer"}}
+				"labels":{"type":"array","items":{"type":"string"},"description":"Label names to resolve and apply. Either labels or label_ids is required."},
+				"label_ids":{"type":"array","items":{"type":"integer"},"description":"Label IDs to apply directly, skipping name resolution. Either labels or label_ids is required."}
 			},
-			"required":["owner","repo","number","labels"]
+			"required":["owner","repo","number"]
 		}`),
 	}
 }