Replace the bridge long-poll with plain polling

[colevels]

tabs serve's long-poll carries ~85 lines of machinery that a 2-second poll
would not need, and the latency it buys is invisible in this product.

The reasoning that led to long-poll was wrong

The justification was "an agent tick costs tokens". It doesn't apply: tabs watch
is already a blocking command the agent calls once. Whether it waits by holding
a socket open or by looping sleep 2 → GET → check internally, the agent sees one
Bash call and one result. Identical token cost.

What long-poll actually buys is sub-second latency instead of ≤2s — on a path where
the extension already debounces 2.5s and the other end is a human clicking a button.

What it costs today

In src/commands/tabs-serve.ts:

• park() / wake() / tabWaiters / verdictWaiters — ~18 lines
• for(;;) loops in GET /tabs and GET /decision, MAX_WAIT_MS, status 499 — ~37 lines
• idleTimeout: 0 — load-bearing and non-obvious. Bun.serve defaults to a
  10-second idle timeout that kills every long-poll; the symptom was tabs watch
  reporting "nothing is listening" about a server that was running fine. Anyone
  editing the Bun.serve options can reintroduce it.

And because a held connection can drop without the server going away, both clients
need to tell those two cases apart — pingBridge() plus a retry loop in
tabs-watch.ts and tabs-suggest.ts, ~30 more lines.

What it becomes

Server handler:

if (tabState && tabState.version > since) return json({ ...tabState, changes })
return new Response(null, { status: 204 })

Client:

for (;;) {
  const res = await fetch(`/tabs?since=${since}`)   // always sub-ms
  if (res.status === 200) return await res.json()
  if (Date.now() >= deadline) return null
  await sleep(2000)
}

Every request is short, so a failure is unambiguously "the server is gone" — the
whole pingBridge retry path can go.

suggest --wait 300 becomes ~150 requests over five minutes instead of one. On
loopback, against a handler that does a map lookup, that is free.

Scope

• src/commands/tabs-serve.ts — delete the waiter machinery; ?wait= becomes a
  no-op accepted for compatibility (or is dropped, since only our own clients send it)
• src/commands/tabs-watch.ts, src/commands/tabs-suggest.ts — internal poll loop,
  drop pingBridge
• The extension is unaffected: it never calls GET /tabs or GET /decision.

Keep the E2E checks from the auto-mode work: watch unblocking on a real tab change,
and suggest returning a Deny with its reason.