feat(computer-use): add native desktop control over MCP

[GrothKeiran]

Summary

• add computer_apps, computer_snapshot, and computer_act as opt-in built-in MCP tools
• provide native Windows capture/input, macOS capture/automation, and Linux X11 adapters with an XWD screenshot fallback
• return screenshots as both standard MCP image content and authenticated short-lived Artifacts
• enforce latest-snapshot binding, serialized action batches, bounds checks, conservative system-key policy, and input cleanup after partial failures
• expose configuration in the Windows/macOS control panels, runtime status, CLI/environment settings, and English/Chinese documentation

Validation

• go test ./...
• go test -race ./internal/tool/computer
• go vet ./internal/tool/computer ./internal/app ./internal/mcp ./internal/config ./internal/desktopruntime ./internal/httpx
• git diff --check
• cross-compiled internal/app for Windows amd64/arm64 and macOS amd64/arm64
• cross-compiled internal/desktopruntime for Windows amd64/arm64
• ran a real Linux MCP end-to-end smoke through initialize, tools/list, computer_apps, computer_snapshot, and a non-mutating computer_act wait action
• Linux smoke returned 11 visible windows, a 1280x800 PNG MCP image, a snapshot id, and an authenticated Artifact

Notes

Windows WPF could not be compiled in this environment because dotnet is unavailable. macOS AppKit and native Windows/macOS permission/input smoke tests still need to run on those operating systems. Real mouse and keyboard mutation was intentionally not exercised on the active Linux desktop; native action dispatch is covered with injected-driver tests, and the MCP action path was exercised using a wait action.