Focus existing Windows Terminal tabs by session - #16

Scope

This PR contains only the Windows Terminal session-focus work split from #15 and targets the maintainer's experimental/windows-terminal-focus branch.

• Capture and restore the exact already-open Windows Terminal window/tab route.
• Focus only that existing tab; never launch wt.exe, create a tab, or resume a CLI.
• Preserve routes across LLMPET restarts and report closed/missing routes without hiding the session menu.
• Keep ordinary tab focus unprivileged.
• Offer elevated-terminal focus as an explicit, default-off tray option with a narrow named-pipe protocol.

Acceptance-point changes

• The registrar captures the real LLMPET user's name, SID, profile, and caller PID before UAC. After UAC—even when different administrator credentials are entered—it validates the still-running caller's owner SID and Windows session, then registers a Limited, interactive logon task for the captured LLMPET SID. The task runs a launcher as that user; only the launcher starts the fixed elevated broker.
• The broker is optional and does not set NSIS perMachine or change the release version.
• A tab route is captured only when the foreground window PID equals the Windows Terminal PID found in the hook's process ancestry, with the same hook-provided WT_SESSION. Background tool hooks reuse the correlated route and cannot cache an arbitrary foreground tab.
• The broker accepts only health, shutdown, and numeric exact-tab focus requests. It validates the named-pipe server executable and Windows session, and validates that the HWND belongs to Windows Terminal.

Validation

• npm test passes on Windows (including protocol input restrictions, disconnect/reconnect, foreground-correlation, cache restoration, missing/closed-tab behavior, and the no-per-click-RunAs guarantee).
• Existing Windows CI now exercises the real isolated scheduled-task registration/query/removal lifecycle when CI=true; it checks the resolved user SID, RunLevel=Limited, interactive logon type, and fixed launcher action.
• scripts/validate-windows-terminal-focus.ps1 produces JSONL evidence for installer, launcher, broker, task lifecycle, reconnect tests, and correlation; optional window/runtime/PID arguments reproduce live exact-tab focus.
• Local Windows validation passed for the full test suite and an isolated real task lifecycle; the validation task was removed afterward.
• All PowerShell helper scripts parse successfully.

No version bump or generated release artifact is included.