chore: state the smoke-helper and CI wall-clock budgets (#2333)

The five rows #2323 left as "assess", each either stated with its
reasoning and measurement, or shared with a constant that already has one.

- F4/F5: `driveAppFlow` and `connectViaDeepLink` default to
  `BROWSER_TIMEOUTS.ui` / `.roundTrip` instead of restating 30_000 /
  45_000 by hand; values unchanged, now one place to raise. A test in each
  helper pins the defaults.
- F1: `announced-child.mjs` states `DEFAULTS` ({ timeoutMs: 30_000,
  pollMs: 250 }) with the measurement — boot-to-announce of the composable
  test server is 135ms median at load average 15, and a passing start pays
  one poll interval, not the budget.
- F7: `pty.mjs` names `SCRIPT_PROBE_TIMEOUT_MS` (5s; the probe spawns in
  1.3ms median, 3.6ms max under load) and stops folding ETIMEDOUT into
  "no script(1)": a starved probe now fails `smoke:tui` loudly instead of
  turning it into an exit-0 skip.
- G1: every job in `.github/workflows/main.yml` declares
  `timeout-minutes` as a hung-job guard — roughly twice the slowest run
  observed over the last 40 push runs (and the last three releases for the
  publish jobs), rounded up to the next five minutes, with the range stated
  per job.

AGENTS.md's timeout section now points at where these live instead of at
this issue.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PsmUAZ6aLUAFqenjJKSSRp
Signed-off-by: cliffhall <cliff@futurescale.com>

scripts/lib/announced-child.mjs

@@ -23,6 +23,34 @@
 import { spawn } from "node:child_process";
 import { setTimeout as delay } from "node:timers/promises";
 
+/**
+ * The readiness budget and poll interval, stated once (#2333) — the same shape
+ * as `render-smoke.mjs`'s `DEFAULTS`, and for the same reason: a budget that
+ * lives in a parameter default is one nobody revisits.
+ *
+ * `timeoutMs` is a ceiling on a poll, not a sleep: the wait returns on the
+ * first poll that sees the announcement, so a passing start pays only the
+ * child's real boot time plus, at most, one `pollMs`. Every smoke that needs a
+ * test server goes through this, which makes it the widest of the smoke
+ * budgets — and the one with the most headroom. Measured on the composable
+ * test server (`server-composable.js --config mcp-app-http`) through this
+ * helper with a 25ms poll, ten starts on a machine at load average 15: boot to
+ * announce was 135ms median, 137ms max. 30s is more than two hundred times
+ * that; what it has to absorb is not a slow server but a machine several
+ * gates deep, where a Node process can take seconds to reach its first line.
+ *
+ * `pollMs` is the passing path's cost. The same measurement at the default
+ * interval landed at 251–254ms every run, which is one interval exactly: the
+ * child announces well inside the first poll, so a start pays ~250ms of which
+ * ~115ms is waiting to look. That is under a second across a whole gate, so
+ * it is stated rather than tuned — lowering it buys nothing anyone would
+ * notice and costs a tighter scan loop on the child's output.
+ */
+export const DEFAULTS = Object.freeze({
+  timeoutMs: 30_000,
+  pollMs: 250,
+});
+
 /**
  * @param {object} opts
  * @param {string} opts.command       Executable to spawn.
@@ -34,8 +62,8 @@ import { setTimeout as delay } from "node:timers/promises";
  *   Called synchronously with the child immediately after spawn, before any
  *   waiting. This is the caller's teardown handle.
  * @param {string} opts.what          Noun used in error messages ("MCP test server").
- * @param {number} [opts.timeoutMs]   Readiness budget (default 30s).
- * @param {number} [opts.pollMs]      Poll interval (default 250ms).
+ * @param {number} [opts.timeoutMs]   Readiness budget (default `DEFAULTS.timeoutMs`).
+ * @param {number} [opts.pollMs]      Poll interval (default `DEFAULTS.pollMs`).
  * @returns {Promise<{ child: import("node:child_process").ChildProcess, match: RegExpMatchArray }>}
  */
 export async function startAnnouncedChild({
@@ -45,8 +73,8 @@ export async function startAnnouncedChild({
   pattern,
   onSpawn,
   what,
-  timeoutMs = 30_000,
-  pollMs = 250,
+  timeoutMs = DEFAULTS.timeoutMs,
+  pollMs = DEFAULTS.pollMs,
 }) {
   const child = spawn(command, args, {
     cwd,