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.

AGENTS.md

@@ -87,9 +87,10 @@ no coverage, about a second. `make precommit` is the same target. Per app,
 warnings fatal, format, test. Run these as often as you like while working.
 
 **Lens pass (every push, and before handing off).** `make verify` runs `check`
-first (cheapest), then each app's heavy sensors - sobelow, coverage, CRAP,
+first (cheapest), then per app: one suite run that exports its coverage, then
-mutation, as those yaks land - and finally both test suites. Measured 2026-10-26:
+that app's sensors (sobelow, coverage, CRAP, mutation, as those yaks land), which
-~10s wall (app 178 tests + blogex 156 tests).
+run cheapest-first and read the export rather than re-running the suite. Measured
+2026-11-01: ~11s wall (app 178 tests + blogex 156 tests + coverage floor).
 
 `.githooks/pre-push` runs `make verify` and blocks the push when anything fails.
 Install it once per clone:
@@ -115,7 +116,10 @@ work mid-feature: finish and commit what you are building, then run it.
 - Adding a sensor: implement it as its own make target, add it to that app's
   `VERIFY_SENSORS` in `app/Makefile` / `blogex/Makefile` (cheapest first), and
   update the slot comments there. A gate that reports a check it did not run is
-  worse than no gate - the `VERIFY_SENSORS` list is what gets echoed.
+  worse than no gate - the `VERIFY_SENSORS` list is what gets echoed. Sensors run
+  *after* the app's `test.coverage` step, because the coverage export they read is
+  produced by that suite run; if a check can pass without its input, make it fail
+  loudly (see the `coverage` target's guard).
 
 ### Triage classes
 

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
+# The Lens pass: everything `check` does, plus, per app, one suite run (which
-# suites, cheapest first. `.githooks/pre-push` runs this before every push (see
+# exports its coverage) and that app's sensors reading the export. Sensors run
-# `make hooks`); run it yourself before handing off. Bypass deliberately with
+# cheapest-first among themselves. `.githooks/pre-push` runs this before every
-# `git push --no-verify`.
+# 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