fix: align auditor channel perms, extend desk gate to tests, drop stale usage-event doc

- permissions: the Auditor is a silent, read-only observer with no say/dm in
  its verb surface, so can_write_channel now returns False for it — matching
  the role's real capabilities instead of granting an unreachable channel
  write (test updated to assert read-only).
- Makefile: make lint and make gate now type-check mypy roboco/ tests/, matching
  make quality / make quality-fast, so the developer-desk gate also catches test
  type errors before submit (tests/ is already mypy-clean).
- docs: CLAUDE.md no longer lists USAGE_UPDATE — only USAGE_SNAPSHOT is published
  to /ws/system.

Makefile

@@ -156,7 +156,7 @@ restart: stop start-example
 lint:
 	@echo 'Formatting w/ Ruff...' ; echo '' ; uv run ruff format .
 	@echo '' ; echo '' ; echo 'Linting w/ Ruff...' ; echo '' ; uv run ruff check .
-	@echo '' ; echo '' ; echo 'Type checking w/ Mypy...' ; echo '' ; uv run mypy roboco/
+	@echo '' ; echo '' ; echo 'Type checking w/ Mypy...' ; echo '' ; uv run mypy roboco/ tests/
 	@echo '' ; echo '' ; echo 'Finding dead code w/ Vulture...' ; echo '' ; uv run vulture vulture_whitelist.py
 
 # Fix code
@@ -285,7 +285,7 @@ quality-fast:
 gate:
 	@uv run ruff format --check .
 	@uv run ruff check .
-	@uv run mypy roboco/
+	@uv run mypy roboco/ tests/
 	@uv run xenon --max-absolute B --max-modules A --max-average A roboco/
 
 # Run all analysis tools