Add pre-push coverage regression check

Add a `pre-push` hook that runs `just test --coverage` locally and fails if any stack's line coverage drops below the current `main` baseline (tolerance: 0.1%).

Depends on #1 (Codecov) to provide a queryable source of the `main` baseline without requiring contributors to set anything up manually.

Start by examining how the repository defines pre-push hooks and how `just test --coverage` reports line coverage for each stack. Investigate dependency #1 and Codecov's queryable `main` baseline, then verify that the hook applies the 0.1% tolerance and fails when any stack regresses.

Written by the indexing model from the issue text.