fix(watchdog): signal-0 liveness, tick-scaled respawn guard, windowsHide

Three-bug chain behind the Windows terminal-agent leak (console window
strobing every 60s, one orphaned agent per watchdog tick until the box
ran out of committable memory):

1. isProcessAlive shelled out to `tasklist /FI "PID eq <pid>"` on Windows
   with a 3s timeout. A Bun.spawnSync that hits its timeout still RETURNS
   with partial stdout, so the `.includes()` PID match read a LIVE agent
   as dead — killAgentByRecord skipped the kill, the watchdog respawned
   around the survivor, and every orphan slowed the next tasklist enough
   to produce the next false negative. Now: `process.kill(pid, 0)` on
   every platform (Node and Bun both map signal 0 to an OpenProcess
   existence check on Windows), with EPERM counted as alive. No
   subprocess, no timeout, no console window.

2. The respawn circuit-breaker was mathematically unreachable — verified
   in this tree: RESPAWN_GUARD_WINDOW_MS was a fixed 60_000 against a
   60_000ms default tick, and each tick pushes at most one respawn
   timestamp, so three pushes span ~120s and can never coexist inside a
   60s window (eviction is strict `>`, and setInterval drift plus
   per-tick work always ages the prior entry past the boundary). The
   guard could not fire at the default tick rate and a steady
   one-per-tick leak ran unbounded. The window now scales with the tick:
   max(60_000, tick * (RESPAWN_GUARD_MAX + 2)), so "3 crashes in quick
   succession → stop" holds at any tick value.

3. The tasklist probe popped a visible console per tick (no windowsHide).
   Removing the shell-out kills that site; the agent-spawn site itself
   already passes windowsHide: true (landed with the bun-polyfill
   windowsHide commit — PR #2414's terminal-agent-control.ts hunk is
   reconciled there rather than duplicated).

New browse/test/process-liveness-windows.test.ts pins all three: no
subprocess from the probe, a static tripwire against reintroducing
`tasklist` + `PID eq` liveness checks in src/, the spawnTerminalAgent
windowsHide + stdio contract, and the window-derived-from-tick
arithmetic. terminal-agent-watchdog.test.ts test 4 now pins the
window/tick relationship instead of the fixed literal that let this
ship. Also converts `new URL(import.meta.url).pathname` to
`import.meta.path` across the static-grep tests it touches — the
pathname form yields /C:/... on Windows and breaks path.resolve.

Contributed by @SYKhayyat (PR #2414).

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

test/hermetic-wiring.test.ts

@@ -19,7 +19,7 @@ import * as path from 'path';
 import * as os from 'os';
 import { getHermeticDirs, hermeticSkillsConfigDir } from './helpers/hermetic-env';
 
-const ROOT = path.resolve(new URL(import.meta.url).pathname, '..', '..');
+const ROOT = path.resolve(import.meta.path, '..', '..');
 
 const RUNNERS = [
   'test/helpers/session-runner.ts',