`pr_checks` runs the full matrix on every PR. #3609 touched only
`apps/webapp/app/routes/admin.tsx` and still ran the 4-job CLI e2e
matrix and 5-job sdk-compat suite.

Adds a `changes` job using `dorny/paths-filter` and gates each tier:

- webapp + e2e-webapp: `apps/webapp/**`, `packages/**`,
`internal-packages/**`
- packages: `packages/**`
- internal: `internal-packages/**` + `packages/**` (cross-deps)
- e2e (cli-v3): `packages/{cli-v3,build,core,schema-to-json}/**`
- sdk-compat: `packages/{trigger-sdk,core}/**`

`.configs/**`, `package.json`, `pnpm-lock.yaml`, `pnpm-workspace.yaml`,
`turbo.json` are also included in every filter since they affect the
whole workspace.

Inlines the `units` reusable-workflow children so each can be gated
independently (status check names also flatten from `units / webapp /
...` to `webapp / ...`). `unit-tests.yml` is unaffected - still used by
`publish.yml`.

Adds an `all-checks` gate that always runs and short-circuits to success
when every dependent is success-or-skipped. With this in place a single
required status check (`All PR Checks`) is enough; before this,
`paths-ignore` would have left required checks Pending on docs/changeset
PRs ([gh
docs](https://docs.github.com/en/actions/managing-workflow-runs/skipping-workflow-runs)).