feat(desktop): minimize exact native windows

[iamnbutler]

Agents can minimize one native window with desktop_minimize, using its unchanged target from desktop_windows. The existing signed native path binds the action to the observed process generation, window ID, bounds, and minimized state; it confirms a change or an already-minimized no-op and preserves uncertain delivery without retrying.

Successful minimize returns fresh window inventory without attempting to capture the minimized window. Restoring it is a separate explicit desktop_restore action with the refreshed target.

Related to #8 and #73.

Validation at 1d0f328:

• bun types, bun run ci, git diff --check, Swift syntax checks, the signed desktop build, and deep/strict signature verification pass. GitHub CI is green.
• Real AppKit window: stale generation, stale bounds, and pre-cancelled requests refused before any minimize callback. Minimize confirmed one background Accessibility write and fresh minimized inventory with no screenshot; minimizing the refreshed already-minimized target confirmed no change and dispatched nothing. Explicit restore succeeded.
• One real pi run called desktop_minimize once. A different worker reopened the same store with identical result/usage and no new model/tool execution or minimize callback. Explicit restore succeeded again. All owned fixture, app, host, and worker processes exited; the temporary channel/project, listener, and native socket were removed.

The live cancellation check covers pre-dispatch cancellation, not an in-flight cancellation claim.

Built in Ace

Used Codex's terminal and file tools for implementation, source review, and validation under Nate's explicit temporary exception to Ace-only development while completing computer use in #8. Live validation used Ace's actual native bridge and pi channel worker with a real provider; no mocked providers or alternate input path. The ongoing dogfooding gap is tracked in #5 and #8.