fix(contrib/windows): the supervised bridge runs headless, not in a terminal tab

[erwin-wee]

What

Register-CollieTask registers the supervised bridge as Execute = powershell.exe under an
Interactive principal. That allocates a real console, and on Windows 11 the default console host is
Windows Terminal — so the bridge that is meant to run in the background shows up as a tab titled
C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe.

It isn't only cosmetic. The tab owns the launcher, so closing it kills the bridge; RestartCount 999
/ RestartInterval PT1M then brings both the bridge and the tab back about a minute later. From the
operator's side it reads as a terminal window that refuses to stay closed.

This wraps the same command in conhost.exe --headless, which runs it on a pseudoconsole with no
window at all.

Why this shape

• The launcher doesn't move. conhost --headless execs powershell as its child, so the $PID
  that _exec-bridge writes to $script:PidFile is still the powershell process, the
  Contains($controlScript) / Contains("_exec-bridge") check in Stop-RecordedCollieProcesses
  still matches it, and taskkill /PID … /T /F still reaches bun through it. conhost exits with its
  child, so Task Scheduler's restart policy is unaffected.
• Not -WindowStyle Hidden — the console is allocated before the script can hide it, so it
  flashes.
• Not the task's Hidden setting — that hides the task from the Task Scheduler UI, not the
  window.
• Not an S4U / session-0 principal. It would also remove the window, but it changes where the
  bridge runs; the Interactive principal is load-bearing for reaching Herdr's socket in the user
  session and resolving bun from the user's PATH.
• Falls back to the previous bare action when conhost.exe is absent.

Verified on

Windows 11 Pro 26200, Windows Terminal 1.24, Herdr plugin herdr.collie, PowerShell 5.1 launcher.
Before: WindowsTerminal.exe → powershell.exe → bun.exe with a visible tab. After:
conhost.exe --headless → powershell.exe → bun.exe, no window handle on any process in the chain,
bridge reachable as usual. contrib\windows\collie-ctl.test.ps1 passes (OK Windows lifecycle tests); two assertions added for the wrap.

Notes

• Per CLAUDE.md, no version bump and no CHANGELOG.md entry — fork PR. Suggested line if you want
  one: Fixed — Windows: the supervised bridge no longer opens a Windows Terminal tab (conhost --headless).
• Happy to add an .adr/ entry if you think "why not -WindowStyle Hidden / S4U" is the kind of
  question that will come back; I left the reasoning as a comment at the registration site instead.