Projects sync registers folders but leaves tasks empty

[pixie-muon]

Problem

project add currently configures a Syncthing folder only. It does not select the Codex tasks whose working directory belongs to that project, so a target Codex desktop can show the project root with No tasks. Imported Windows task paths are also copied without separator-aware mapping, which can prevent correct grouping on macOS.

Expected behavior

• Discover project roots already known to Codex desktop.
• Select all non-archived tasks under an explicitly selected project.
• Publish a portable per-device project catalog.
• Accept the source folder ID on a target device and record source-to-local path mapping.
• Register the accepted local root in Codex desktop.
• Preserve existing Sync2 safety boundaries and avoid copying live databases.

Acceptance criteria

• Project-level task selection is explicit and test-covered.
• Windows/macOS path mapping is separator- and boundary-aware.
• Target import stores the mapped local cwd.
• Documentation and changelog describe the new workflow.
• npm run check passes.