Both jobs ran on every pull request, so a change to a Rust binary waited on a
Postgres service and a full TypeScript test run, and a change to a TypeScript
route spent a runner compiling Rust. Neither told anyone anything.
A `paths` filter belongs to a workflow rather than to a job, so the two jobs
become two workflows. That is the whole cost of the change: both job bodies are
carried over unchanged, and only the triggers differ.
The filters are written from what each job actually reads. `packages/` is
entirely TypeScript so it is taken whole, and the two TypeScript apps are named
because the rest of `apps/` is Rust. The Rust job takes its own directory plus
the workspace root and lockfile, which pin every version it builds against, and
nothing else — it depends on no other member of the workspace.
The failure mode of a path filter is silence: a job that does not run leaves a
green pull request. So the one thing a future change has to remember is written
where it will be read — adding a TypeScript app means adding it to that list.
