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>

internal/storage/storage.go

@@ -21,6 +21,7 @@ type Storage interface {
 	RemoveDependency(ctx context.Context, issueID, dependsOnID string, actor string) error
 	GetDependencies(ctx context.Context, issueID string) ([]*types.Issue, error)
 	GetDependents(ctx context.Context, issueID string) ([]*types.Issue, error)
+	GetDependencyRecords(ctx context.Context, issueID string) ([]*types.Dependency, error)
 	GetDependencyTree(ctx context.Context, issueID string, maxDepth int) ([]*types.TreeNode, error)
 	DetectCycles(ctx context.Context) ([][]*types.Issue, error)