Run macOS apps on a hosting Mac with stim macos --host #2475

Track C of #2403. Contracts: contracts comment, sections 4 and 5. Shared types: #2472.

Problem

There is no client command that places a macOS app on an approved hosting Mac. No client code drives the device-host.* protocol at all: hosting.machines and stim doctor --fix only obtain the approval. Desktop and the phone talk only to their local stim-server, so they cannot show a hosted session.

Evidence

• packages/stim-cli/src/commands/macos.ts builds and launches locally only.
• The client connection code to reuse is BuildConnection in packages/stim-cli/src/offload/client.ts: a pinned wss connection, hello with the token, and a capability check. Also reuse pinnedEndpoint in offload/tailnet.ts and readDeviceHostMachines in core/state.
• The HostedMacosPlacement / HostedAgentAccess types land in feat: add hosted macOS app contracts #2472.

Fix idea / scope

1. stim macos --host <machine> [--json], a flag approved for this stack by the maintainer's Offload native macOS builds and host app windows on another Mac #2403 request:
   • Resolve the approved credential and pinned endpoint. Refuse anything else, and never fall back to a local launch.
   • Build locally and stage the bundle with its base CFBundleIdentifier.
   • Attach to the session recorded in workspace state, or reserve one with a fresh attempt. Wait for ready.
   • Offer the manifest, upload the missing chunks, launch, and poll app.attach.
   • Record macos.host and print the contract payload. Write agent files only from a non-none grant.
2. stim status and stim stop / worktree remove cover hosted placements. Stop sends device-host.stop and removes the agent files.
3. A client stim-server relay: for frames.subscribe, control.* and input.* on { workspace, platform: 'macos' } with macos.host, the local server opens the pinned device-host connection and relays. Desktop needs only the existing macOS viewer route. The phone gets the same route through its paired server, if cheap.
4. Update the guide (stim guide agent routing plus the macOS topic) and website docs. Agents stop using mini-desktop.sh once Host prebuilt macOS apps in owned device-host sessions #2473, Stream and control hosted macOS apps #2474 and Run macOS apps on a hosting Mac with stim macos --host #2475 land.

Acceptance

• Unit tests with a fake host connection: reserve, deliver, launch and stop; an unapproved or changed node is refused; JSON stdout stays a single payload.
• A real two-Mac run (MacBook client, mini host, tailnet only, no SSH): build, launch, view in Desktop, click, stop.

Out of scope

Remote builds (#2476) and the agent driver (#2477).

Depends on #2473 for the end-to-end run. CLI and relay plumbing can start now against the contracts with a fake host.