docs: the Lens pass runs each suite before its sensors

Same shape in both apps now that coverage is wired: one suite run exports the
coverage, then the sensors read it. AGENTS.md said "sensors ... and finally both
test suites" and told sensor authors only to list targets in VERIFY_SENSORS; add
the export dependency and the "a check must fail when its input is missing" rule,
since `mix test.coverage` exits 0 when it finds no export.

Makefile

@@ -17,10 +17,11 @@ check:
 # Precommit target for CI/pre-commit hooks
 precommit: check
 
-# The Lens pass: everything `check` does, plus the heavy sensors and both test
-# suites, cheapest first. `.githooks/pre-push` runs this before every push (see
-# `make hooks`); run it yourself before handing off. Bypass deliberately with
-# `git push --no-verify`.
+# The Lens pass: everything `check` does, plus, per app, one suite run (which
+# exports its coverage) and that app's sensors reading the export. Sensors run
+# cheapest-first among themselves. `.githooks/pre-push` runs this before every
+# push (see `make hooks`); run it yourself before handing off. Bypass
+# deliberately with `git push --no-verify`.
 verify: check
 	@echo "==> verify: app"
 	@$(MAKE) -C app MISE_BIN=$(MISE_BIN) verify