chore(ci): drop npm leftovers, switch CI to bun
Removes the dead `.npmrc` (only set `ignore-scripts=true`, which bun
doesn't honor) and migrates the lint and tests workflows from
`actions/setup-node` + `npm` to `oven-sh/setup-bun` + `bun`. Adds
`--frozen-lockfile` so a drifted `bun.lock` fails CI rather than
silently regenerating.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>