fix(browse): isProcessAlive uses signal-0 on every platform — no more tasklist probe (#1952)

Replace the Windows tasklist shell-out in isProcessAlive with
process.kill(pid, 0), unifying all platforms on the POSIX idiom. Node maps
signal-0 to an OpenProcess existence check on Windows — and the Windows
daemon runs under Node (dist/server-node.mjs + bun-polyfill, the documented
oven-sh/bun#4253 fallback) — so the probe is portable.

Why the shell-out had to go, beyond the cosmetic conhost flash the watchdog
blinked into the foreground every 60s (#1952): a Bun.spawnSync that hits
its timeout still RETURNS with partial stdout, so the `.includes()` PID
match answered "dead" for LIVE processes under load — the false-negative
half of the #2414/#2295 leak chain. Signal 0 spawns nothing, cannot time
out, and is ~5 orders of magnitude faster (measurements in #2414). EPERM
still reports alive (process exists, we just can't signal it).

Layered on the post-#2414-absorb shape: test 3 in
process-liveness-windows.test.ts now asserts the probe is subprocess-free
on ANY platform (win32 exemption dropped), test 4's static tripwire loses
its error-handling.ts exemption (a `tasklist … PID eq` existence probe
anywhere in src/ now fails CI), and windows-spawn-hide.test.ts drops its
tasklist-in-error-handling needle (nothing spawns, which is stronger than
hiding the window).

Tests: process-liveness-windows + windows-spawn-hide + error-handling —
17 pass, 0 fail.

Fixes #1952.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/src/error-handling.ts

@@ -7,8 +7,6 @@
 
 import * as fs from 'fs';
 
-const IS_WINDOWS = process.platform === 'win32';
-
 // ─── Filesystem ────────────────────────────────────────────────
 
 /** Remove a file, ignoring ENOENT (already gone). Rethrows other errors. */
@@ -39,24 +37,32 @@ export function safeKill(pid: number, signal: NodeJS.Signals | number): void {
 /**
  * Check if a PID is alive. Pure boolean probe — never throws.
  *
+ * Signal 0 on EVERY platform (#1952). Node maps `process.kill(pid, 0)` to an
+ * OpenProcess existence check on Windows — and on Windows the browse daemon
+ * runs under Node (dist/server-node.mjs + bun-polyfill, the documented
+ * fallback for oven-sh/bun#4253) — so the POSIX idiom is portable here.
+ *
+ * Windows used to shell out to `tasklist /FI "PID eq <pid>"` and
+ * string-match the CSV. That was wrong in two ways, both hit in production:
+ *
+ *   1. FALSE NEGATIVES UNDER LOAD (#2414/#2295): tasklist takes ~700-1700ms
+ *      on an idle box and far longer under memory pressure. A Bun.spawnSync
+ *      that hits its `timeout` still RETURNS, carrying partial stdout — so
+ *      the `.includes()` match came back false and a LIVE process was
+ *      reported dead. Callers that validate liveness before killing
+ *      (killAgentByRecord, the terminal-agent watchdog) then skipped the
+ *      kill and respawned around the survivor — one leaked terminal-agent
+ *      per tick, self-reinforcing (each orphan slows the next tasklist).
+ *   2. A console window per probe (#1952): the watchdog blinked a conhost
+ *      window into the foreground every 60s for the whole session.
+ *
+ * Signal 0 spawns nothing, cannot time out, and is orders of magnitude
+ * faster (~0.004ms vs ~270ms measured in #2414).
+ *
  * EPERM means the process EXISTS but we lack rights to signal it. That is
- * alive — returning false there makes callers that validate liveness before
- * killing (killAgentByRecord, the terminal-agent watchdog) skip the kill and
- * respawn around a survivor, leaking one process per watchdog tick (#2414,
- * #2295).
+ * alive — returning false there would reintroduce failure mode 1.
  */
 export function isProcessAlive(pid: number): boolean {
-  if (IS_WINDOWS) {
-    try {
-      const result = Bun.spawnSync(
-        ['tasklist', '/FI', `PID eq ${pid}`, '/NH', '/FO', 'CSV'],
-        { stdout: 'pipe', stderr: 'pipe', timeout: 3000, windowsHide: true }
-      );
-      return result.stdout.toString().includes(`"${pid}"`);
-    } catch {
-      return false;
-    }
-  }
   try {
     process.kill(pid, 0);
     return true;