0.15.39: dedicated pi-chrome automation tab/window isolation (#6)

Stop pi-chrome from hijacking the user's active tab. Page/navigation actions
with no explicit target now route to a dedicated automation window owned by the
calling Pi session (falling back to a dedicated tab if a window can't be made),
instead of overwriting whatever the user has open.

- Session-scoped ownership: Map<sessionKey, target> in the service worker, keyed
  by a sessionKey forwarded from index.ts, so concurrent sessions (one shared
  extension) never fight over a window or close each other's tabs.
- Persistence: ownership mirrored to chrome.storage.session so a service-worker
  restart re-hydrates and reuses the window instead of orphaning it. Cleared on
  full browser restart; restored windows are then left alone (never reclaimed).
- getTabByParams default branch resolves the owned target; tab.activate/close/
  group/ungroup pass createOwnedTarget:false so management never auto-creates a
  throwaway tab or falls back to the user's active tab.
- Session-group integration: the dedicated-window tab still joins this session's
  tab group, in its own window; grouping stays best-effort.
- Lifecycle: cleanup on /chrome revoke and real session end (not /reload);
  fire-and-forget so it never blocks /quit, /reload, or shutdown.
- Tests: new test-suite/unit/automation-target.test.mjs (isolation, reuse,
  concurrency, restart/reconnect re-hydration, guarded tab.*, group integration,
  window->tab fallback, robust cleanup). Docs + CHANGELOG updated.

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