Implement bd-10: Export/import dependencies in JSONL

Added dependency support to export/import workflow:

Changes:
- Added GetDependencyRecords() to Storage interface to get raw dependency records
- Extended Issue struct with Dependencies field (omitempty for backward compat)
- Modified export.go to populate dependencies for each issue
- Modified import.go to process dependencies in second pass after all issues exist
- All tests pass

Benefits:
- JSONL is now self-contained with full dependency information
- Enables proper collision resolution in future (bd-12+)
- Idempotent imports: existing dependencies are not duplicated
- Forward references handled: dependencies created after all issues exist

Example output:
{
  "id": "bd-10",
  "title": "...",
  "dependencies": [{
    "issue_id": "bd-10",
    "depends_on_id": "bd-9",
    "type": "parent-child",
    "created_at": "...",
    "created_by": "stevey"
  }]
}

Closes bd-10

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/export.go

@@ -48,6 +48,16 @@ Output to stdout by default, or use -o flag for file output.`,
 			return issues[i].ID < issues[j].ID
 		})
 
+		// Populate dependencies for each issue
+		for _, issue := range issues {
+			deps, err := store.GetDependencyRecords(ctx, issue.ID)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "Error getting dependencies for %s: %v\n", issue.ID, err)
+				os.Exit(1)
+			}
+			issue.Dependencies = deps
+		}
+
 		// Open output
 		out := os.Stdout
 		if output != "" {