Move the embedded desktop tools to the Swift stack (native2) when desktop-tools is ready #532

desktop-tools is replacing its Node client and the vendored Peekaboo with a single Swift codebase, `native2` (githubnext/desktop-tools#143). Ace pins desktop-tools to a commit, `c3e08eb` in the root `package.json` catalog. Keep that pin until the desktop-tools issues below are closed. Pins at or after desktop-tools v0.16.1 still build and run the existing stack by default, so bumping for other fixes is safe as long as nothing below is turned on.

What changes for Ace

Ace touches desktop-tools in four places:

• `apps/desktop/src/native.ts` (C ABI through `bun:ffi`): no change. `native2`'s `libDesktopTools.dylib` exports the same functions with the same arguments, including `desktop_tools_start(socket, client identifier)`.
• `apps/desktop/scripts/helper.ts`: pass `--native2` to `dist/build.js`, then copy the same two outputs (`libDesktopTools.dylib`, and `desktop-tools-client` as `ace-desktop-client`). `sign.ts` signs them as today. The host serves only that client identifier from Ace's team.
• `apps/host/src/desktop.ts`: `createDesktop({ client, socket, wire: 2 })`.
• `packages/channel/src/desktop.ts`: `/protocol` and `/tools` imports stay the same.

Waiting on desktop-tools

• Embedders: ship native2 in the package and build it without Peekaboo desktop-tools#171: ship `native2` in the package, and build it without Peekaboo.
• Embedders: a persistent client mode, so createDesktop keeps one connection open desktop-tools#172: a persistent client, so `createDesktop` keeps one connection open. Without it, Ace pays a client spawn per call, as today. Not a blocker, but it's where the speed gain is: on desktop-tools' bench, `desktop_windows` takes 115 ms on the shipped stack and 6 ms on native2's MCP server.
• Embedders: generate /tools and /protocol from native2's contract desktop-tools#173: `/tools` generated from `native2`'s contract. Until then, Ace's model reads `desktop_launch` and `desktop_open` as foreground with no `activate`, while native2 launches in the background.

Checks before switching

• The lane note on `package.json`: insert into Save sheet fields (desktop_insert can't type into a Save sheet's filename field #390, desktop-tools d1c4304). Run that flow live on native2.
• The lane note on `packages/channel/src/desktop.ts`: `desktop_open` keeps the file-URL query/fragment sentence. It's still present.
• Validate desktop tool use in Ace and track known gaps #427's desktop tool validation, rerun on native2.
• The approval prompt and pause, if Ace uses them. They haven't been run end to end through an embedding app on native2.