feat(scheduler): ScriptRunner through the process spawn policy

Spec: on Windows the command goes through Arguments like the bash tool, since cmd.exe doesn't parse ArgumentList quoting; the workspace is created if missing, and a timeout vs cancellation outcome is spelled out.

docs/superpowers/specs/2026-09-26-scheduler-design.md

@@ -163,16 +163,20 @@ Invalid jobs are dropped with a warning. `SaveAsync(jobs)` writes `jobs.json.<pi
 
 - The shell matches the built-in `bash` tool, so a script the agent tested with `bash` behaves the same when it's
   scheduled: `/bin/sh -c <command>` on Unix and `cmd.exe /d /s /c` on Windows. It's overridable through
-  `SchedulerOptions.Shell`. The command is passed through `ArgumentList`, not quoted by hand.
-- The working directory is the target's workspace. Stdin is closed.
+  `SchedulerOptions.Shell`. On Unix the command is passed through `ArgumentList`, not quoted by hand. On Windows
+  `cmd.exe` doesn't parse `ArgumentList`'s quoting, so the command goes through `Arguments` as
+  `/d /s /c "<command>"`, exactly as the `bash` tool does. A configured `SchedulerOptions.Shell` always uses
+  `ArgumentList`.
+- The working directory is the target's workspace (created if missing). Stdin is closed.
 - The command, cwd and environment come from `IProcessSpawnPolicy.Prepare` (DI singleton, registered by
   `AddAgenticCore`) with `Purpose = ScheduledScript`, `Scope = target`, `Origin = job id` and `TimeZoneId` = the
   target's zone. So the script sees the same filtered environment as `bash`, with `TZ` set to the chat's zone, and
   host hooks apply. There's no scheduler-specific scrub list any more (`SchedulerOptions.ScrubbedEnvironment` is
   removed). A policy that throws is `Failed(null, "process environment hook failed: …")`. Session-level
   `ProcessSpawnEvent` handlers don't run: a job fires without a live session.
-- Timeout or cancellation → `Process.Kill(entireProcessTree: true)`. A shell that fails to start is `Failed(null, "failed to start <shell>: …")`,
-  never an exception.
+- Timeout or cancellation → `Process.Kill(entireProcessTree: true)`. A timeout is `Failed(null, stderrTail, timedOut: true)`;
+  a cancellation throws `OperationCanceledException` after the kill. A shell that fails to start (or a workspace that
+  can't be created) is `Failed(null, "failed to start <shell>: …")`, never an exception.
 - Output capture as in the reference, then `Trim()` on stdout.
 
 ### 4. `JobScheduler` (process-wide, the slice's entry type)