feat: add project export and import functionality

- Implemented `pcli project export` command to export project hierarchy as JSON.
- Added `pcli project import` command to import project data from JSON.
- Created user client to fetch user details for comment attribution.
- Introduced new data structures for export and import processes.
- Ensured name-based references in exports and handled conflicts during imports.
- Added versioning and progress reporting for import operations.
- Updated documentation and specifications for new features.

.claude/skills/openspec-sync-specs/SKILL.md

@@ -6,7 +6,7 @@ compatibility: Requires openspec CLI.
 metadata:
   author: openspec
   version: "1.0"
-  generatedBy: "1.1.1"
+  generatedBy: "1.2.0"
 ---
 
 Sync delta specs from a change to main specs.