feat(hints): add label field to StructuredHint (#216)

Add a short semantic label (e.g. "routes via members", "implementors",
"handler") to every structured hint so LLM agents can triage hints
without parsing args. 37 named LABEL_* constants derived from existing
TPL_* template prefixes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17