[ankurmans]

Runs the whole suite in four parallel shards (node --test-shard) on pull requests that need it and on the push and nightly runs, so a run takes the time of one shard instead of the whole suite. Before: 53 minutes for a pull request that ran everything (PR 36), 100 to 131 minutes for the nightly, 35 minutes for the test step. After: measured on this pull request's own run, which changes .github/ and so runs everything.

• Jobs: lint, plan, test-subset, test (four shards), build, and check, the one name to require (green when every job passed or was skipped on purpose). Push and nightly runs: lint, build, full-suite (four shards) and test-map, which joins the shards' notes and caches the map. Nothing waits for the tests there, as before.
• Concurrency: a new push cancels the older run of a pull request; runs on main (push, nightly, manual) are never cancelled. The group includes the event and, on main, the commit, so a nightly can't queue a push and two pushes don't replace each other.
• Triggers: push only on main (as before). The nightly moves from 05:00 to 03:00 UTC.
• No paths-ignore for markdown: tests read docs, READMEs and AGENTS.md as text, so skipping the workflow would skip checks. scripts/affected-tests.mjs already runs only the tests that read a changed document, none for one nothing reads.
• Nothing installs system packages or uses sudo; setup-node caches npm. No check was weakened or removed.