fix(engine): report real projects for paired machines, not install folders or / #3425

Problem

list_machines (orchestration v12) reports projects for each paired machine that are not projects:

• A Windows machine lists the app's install folder twice, once in verbatim form:
  "projects": ["RustCode", "RustCode"], and start_session on it says
  has no project at …; pass project as one of: \\?\C:\Users\<user>\AppData\Local\RustCode, C:\Users\<user>\AppData\Local\RustCode.
  Some threads there recorded their working directory through std::fs::canonicalize, which on Windows returns the \\?\ verbatim form, so the same folder counts twice. The folder itself is where the app was started, not a project the user chose.
• A Mac with the app closed lists "projects": ["/"]. start_session with machine and no project started a thread there in / (the server's working directory): the agent reported "Working directory: /".

Acceptance criteria

• Paths reported to other machines and stored for threads drop the \\?\ prefix (keep \\?\UNC\ as \\), so one folder is one project.
• A thread started on another machine without project starts in that machine's home folder (or is refused with the list of its projects), never in / or the app's install folder.
• list_machines lists the user's projects on that machine, not the working directories of threads started without one.