Machines: create and edit Projects on another Machine from the app #68

Follow-up to #66 (PR #67). #66 shows another Machine's Projects and controls their Services. This issue covers creating and editing Projects on a Machine from the app, so a developer never needs to sit at it.

What it looks like

Add Project › Runs on: This Mac | devbox. When a Machine is picked:

• The folder picker becomes a remote folder browser. It starts in the Machine's home folder, with breadcrumbs and a path field that autocompletes. It only reads.
• The scan runs on the Machine. Its core has the detectors, so the review sheet and its pre-checked Suggestions work as they do for a local folder.
• The Project is saved on the Machine, in its own Project Configuration. Agents there (localhost mcp) see it at once, which keeps one truth.
• Editing Services, Dependencies, variables, Environment Profiles and Removing all work as they do for this computer's Projects.
• Secrets typed in the app go to the Machine's secret store over the SSH connection. Their values never come back.

Words: "Machine" and "Runs on", never "environment", which already means variables and Environment Profiles.

How

• New Machine protocol requests, additive only, behind MACHINE_PROTOCOL_VERSION 2:
  • list a folder
  • scan a folder
  • add, update and remove a Project or Service
  • set and remove a Secret
• Every change is made as Origin::Machine.
• An older Machine answers Unsupported, and the app says that Machine's localhost needs updating.
• The app hides editing for Machines on protocol 1.

Risks

• Headless Machines often have no Secret Service running, or it's locked. Setting a Secret must then fail with a clear LocalhostError that says what to do, never fall back to storing it in plain text.
• Two people or agents may edit the same Project at once. The Machine's ProjectLibrary decides, and every viewer gets the result through the existing Projects message.

Done when

• Protocol requests and relay handlers, with tests over a duplex and over the private-sshd tests
• FFI, and the macOS UI: the Runs on picker, the remote folder browser, the scan review, and editing
• The same in the Tauri app
• Docs: configuration docs, AGENTS.md Vocabulary (Machine, Forward) and Decisions