Make packaging smoke fail-safe against event-loop drain; pin Node < 26

[tadaspetra]

What changed

scripts/package-smoke.mjs (the package:smoke step of pnpm run check) could exit 0 without packaging anything. Under Node 26, @electron/packager's internal extract-zip call stalls while extracting the Electron zip without ever rejecting, so Node's event loop drains and the process exits 0 — no success log, no error, a silent false green in the required verification suite. The same stall breaks the electron package's install script (path.txt is never written), making fresh installs unusable on Node 26.

• Fail-safe runner (scripts/package-smoke-run.mjs): process.exitCode = 1 is preset up front and only reset to 0 after verified success, a beforeExit guard prints a drain diagnostic if the packager promise never settles, and success now requires the dist-smoke output directory to exist and be non-empty — a resolved packager promise alone is not proof. package-smoke.mjs is now a thin entry over the runner.
• Tests: unit tests for the runner (tests/unit/package-smoke-run.test.mjs) and integration tests (tests/integration/package-smoke-exit.test.mjs) that spawn real child processes wired exactly like the production entry, proving actual process exit codes for hang (drain) / success / empty-output / rejection.
• Node pinning: .nvmrc (22) and engines: ">=20.19 <26" in package.json; CI's setup-node now reads node-version-file: '.nvmrc'. The failure mode is documented in AGENTS.md and docs/production/runbook.md.

Verification

• Demonstrated the real failure is caught: on Node 26.7.0, pnpm package:smoke now exits 1 with Packaging smoke did not complete: the event loop drained before packaging finished… (previously: silent exit 0).
• Full pnpm run check passes under Node 22.23.2 (checksum-verified toolchain), including a genuine Packaging smoke succeeded — run on this branch after fast-forwarding to latest main.
• The broken local electron install (missing path.txt from the same drain) was repaired by re-running its install script under Node 22.

Reviewer notes

• The drain guard relies on beforeExit not firing before the packager settles in the healthy path; the integration tests cover both directions with real processes.
• engines is advisory under pnpm's defaults (warning, not error); the hard protection is the smoke script itself now failing loudly.

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