Add closed_at timestamp tracking to issues

- Add closed_at field to Issue type with JSON marshaling
- Implement closed_at timestamp in SQLite storage layer
- Update import/export to handle closed_at field
- Add comprehensive tests for closed_at functionality
- Maintain backward compatibility with existing databases

Amp-Thread-ID: https://ampcode.com/threads/T-f3a7799b-f91e-4432-a690-aae0aed364b3
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/schema.go

@@ -17,7 +17,8 @@ CREATE TABLE IF NOT EXISTS issues (
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     closed_at DATETIME,
-    external_ref TEXT
+    external_ref TEXT,
+    CHECK ((status = 'closed') = (closed_at IS NOT NULL))
 );
 
 CREATE INDEX IF NOT EXISTS idx_issues_status ON issues(status);