feat(hints): add label field to StructuredHint (#217)

* 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>

* test(hints): add label assertions and remove unused constants

- Add label parameter to _assert_structured_hint helper
- Assert all structured hints have non-empty labels in parity + round-trip tests
- Add test_structured_hint_label_values with 13 specific label checks
- Remove 3 unused LABEL_* constants (role filter fallback, role collision,
  has unresolved) — will be added when structured counterparts are built

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

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: HumanBean17