Setup from the app, one Drive folder per Mac, README for Homebrew

[mbeczynski]

Built on #9 (translation). Merge #9 first; this PR then retargets to main automatically.

One Drive folder per Mac

Until now, the folder and the image were the constant mac-studio on every Mac. Two Macs on one Google account would have shared one image, because drive.file access is per OAuth client, not per token.

• Where the name lives: each Mac stores its folder name in ~/Library/Application Support/CloudMachine/drive-folder.
• When it is chosen: configure-remote decides it once, before creating the rclone remote. The default comes from the computer name, or set it with --folder NAME.
• Existing installations: a Mac whose rclone remote already exists predates this setting and keeps mac-studio. That is this Mac: drive-status shows Drive folder: gdrive:CloudMachine/mac-studio. With no file at all, the legacy name is used too.
• No switching: a stored or legacy folder is never switched to another name. That would start an empty backup and orphan the old one, so the request is refused.
• DriveFolderTests cover every branch, including the refusals.

Setup from the app

The Required Setup Steps card used to list only commands to copy. The controller's install, create and attach actions were connected to nothing. The card now shows the next steps in order, each with a button:

1. Install rclone, Install FUSE-T.
2. Connect Google Drive: command to copy (OAuth waits for the browser in Terminal).
3. Open Full Disk Access settings.
4. Install agents.
5. Create image (with a size field), then Attach image.
6. Point Time Machine at CloudMachine: sudo command to copy.

Google comes before anything that mounts, because that is where the folder is chosen. State not measured yet never produces a step. SetupPlan is a pure function, covered by SetupPlanTests.

README

• Homebrew only: Getting started → open the app → follow the card. Includes several Macs on one account, and upgrading/uninstalling.
• Running it: starts with the menu bar and the window; the CLI follows.
• Terminal setup stays as the alternative, in an order that works. The old README ran create-image before install-launchd, but create-image refuses without the mount, which install-launchd starts.
• Moved out: building from source, local installs, tests, releases and the harnesses → docs/building.md.
• Cask caveats point to the app and #getting-started.

Checks

• swift test: 323 tests, 0 failures. swift format lint --strict passes.
• Cask template: brew style passes.
• Not run here: the GUI buttons themselves. This Mac runs the production instance, and the setup actions install agents and create images. They call the same Core functions as the CLI subcommands. Worth one run-through on a clean Mac or a second macOS user before the first release.

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