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/types/types.go

@@ -8,20 +8,21 @@ import (
 
 // Issue represents a trackable work item
 type Issue struct {
-	ID                 string     `json:"id"`
-	Title              string     `json:"title"`
-	Description        string     `json:"description"`
-	Design             string     `json:"design,omitempty"`
-	AcceptanceCriteria string     `json:"acceptance_criteria,omitempty"`
-	Notes              string     `json:"notes,omitempty"`
-	Status             Status     `json:"status"`
-	Priority           int        `json:"priority"`
-	IssueType          IssueType  `json:"issue_type"`
-	Assignee           string     `json:"assignee,omitempty"`
-	EstimatedMinutes   *int       `json:"estimated_minutes,omitempty"`
-	CreatedAt          time.Time  `json:"created_at"`
-	UpdatedAt          time.Time  `json:"updated_at"`
-	ClosedAt           *time.Time `json:"closed_at,omitempty"`
+	ID                 string         `json:"id"`
+	Title              string         `json:"title"`
+	Description        string         `json:"description"`
+	Design             string         `json:"design,omitempty"`
+	AcceptanceCriteria string         `json:"acceptance_criteria,omitempty"`
+	Notes              string         `json:"notes,omitempty"`
+	Status             Status         `json:"status"`
+	Priority           int            `json:"priority"`
+	IssueType          IssueType      `json:"issue_type"`
+	Assignee           string         `json:"assignee,omitempty"`
+	EstimatedMinutes   *int           `json:"estimated_minutes,omitempty"`
+	CreatedAt          time.Time      `json:"created_at"`
+	UpdatedAt          time.Time      `json:"updated_at"`
+	ClosedAt           *time.Time     `json:"closed_at,omitempty"`
+	Dependencies       []*Dependency  `json:"dependencies,omitempty"` // Populated only for export/import
 }
 
 // Validate checks if the issue has valid field values