Global WebSocket used on a Node 20 minimum — third independent hit today; needs a guard, not a third fix

[pseudoseed]

packages/codev/src/agent-farm/lib/tunnel-client.ts:500 uses the global WebSocket:

ws = new WebSocket(buildTunnelWsUrl(this.options.serverUrl));

packages/codev/package.json declares engines.node: ">=20.0.0", and Node 20 has no global
WebSocket. Measured on this machine, which runs the declared minimum:

$ node -e 'console.log(process.version, typeof WebSocket)'
v20.19.2 undefined

So this line throws ReferenceError: WebSocket is not defined on the project's own minimum
supported runtime. ws: ^8.18.0 is already a dependency of packages/codev, so the fix costs
nothing new.

This is on main today and is not introduced by any open PR.

Why it is being filed now

The same bug has now been hit three times independently, by three different agents, in three
different files in one day:

1. spec-146-phase-9-live-harness.test.ts — surfaced the moment the test stopped being skipped
   (PR [Air #180] Pin the live t3 server runtime #191). Fixed there by using the package's existing ws.
2. agent-farm/thread-backend.ts:91 — found by the codex lane in spec 146 phase 9's iteration-2
   review. Being fixed on that branch.
3. This one, found by sweeping for the pattern after the second hit.

Three independent discoveries of one root cause is the signal that the pattern needs a guard,
not three fixes. Nothing currently stops a fourth.

Ask

1. Replace the global with the ws package here.
2. Add a lint rule or a test that fails on new WebSocket( outside a ws import, across
   packages/*/src. A grep-based test in the style of the existing boundary tests is enough —
   the point is that the next occurrence is caught by CI rather than by whichever agent happens
   to run on Node 20.
3. Any such test must assert its own reach: it should fail if pointed at a directory that does
   not exist, so a path typo cannot make it vacuously green.

Note the asymmetry that makes this easy to miss: on Node 22+ every one of these works fine. The
failure is invisible to anyone not running the declared minimum, which is exactly the runtime the
project promises to support.

Related: #190, #193 (guards that cannot report their own reach).