ci(test): run the suite and the typechecker, not just the build (MX-222)

[MGrin]

The only CI job here was managed-install. It proves the plugin builds from a git install with runtime deps only — a real, non-obvious check that a runtime import parked in devDependencies would fail. It stays untouched.

It is not a check on whether the code works. bb plugin build bundles without typechecking, and npm test ran only when somebody typed it, so a change breaking all 269 tests still got a green check on its PR.

This adds a second job, deliberately separate rather than more steps on the first: managed-install installs --omit=dev, so typescript and @types/* are gone by the time it ends, and a build failure and a test failure should stay distinguishable at a glance.

Verified locally at this commit: npm ci, npm run typecheck, npm test — all exit 0, 269 pass / 0 fail.

What a green Test run does NOT mean. No test imports server.ts (1678 lines), so the plugin wiring, handlers and schedules are typechecked and never executed. That is the same hole MX-219 measured in bb-plugin-bus, where deleting a call left 23/23 passing. The workflow comment says so, in the file, so the next reader of a green check is not misled by it.

Swept across the siblings in the same pass: bb-plugin-bus already had this (MX-228) and gets only an npm install → npm ci alignment; bb-plugin-system has no tests at all and gets a Typecheck job named for what it actually checks.

[MGrin]

Negative control — the new job was observed going red, then the branch was deleted.

A gate that has never failed is not known to be a gate. #20 (now closed, branch gone) added one deliberately failing assertion to lib.test.ts and nothing else:

job	result	run
Test — unit tests and typecheck	fail	32442705108 — not ok 270, # fail 1, exit 1
Managed install	pass	32442705101

That row is the whole ticket: with a broken test in the tree, the old job is green. It is not lying — it is answering a different question — but a reader who saw only that check would have merged it.

This branch, unbroken: Test 32442564874 — # tests 269, # pass 269, # fail 0 — and Managed install 32442564853, both pass.