Fix project task synchronization across devices

[pixie-muon]

Closes #5

What changed

• discover Codex desktop project roots and their non-archived tasks
• make project add/project select select every task whose cwd belongs to the chosen project
• publish per-device portable project catalogs with folder IDs, source paths, Syncthing device IDs, and task lists
• add target-side project accept, explicit project map, and Codex desktop registration
• make Windows/macOS path mapping separator-aware and path-boundary-aware
• document the project workflow and add paired changelog entries

Why

The previous implementation synchronized only the project folder registration. A target Codex desktop could therefore show a project such as MUON with No tasks. Imported Windows paths could also fail to group under the macOS project root.

Impact

Selecting a project now carries its stable Codex task histories as a group, while preserving Sync2 explicit selection, append-only conversation, and no-live-database-copy safety boundaries. Target devices can accept the original folder ID and map task metadata to their own local path.

Validation

• npm run privacy:scan
• npm run docs:check
• npm test (70 checks)
• npm run check
• git diff --check