fix: priority:0 omitempty and markdown nil pointer (GH#671, GH#674)

- Remove omitempty from Priority field so P0 issues preserve priority:0
- Add nil check for store in createIssuesFromMarkdown

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/types/types.go

@@ -17,7 +17,7 @@ type Issue struct {
 	AcceptanceCriteria string         `json:"acceptance_criteria,omitempty"`
 	Notes              string         `json:"notes,omitempty"`
 	Status             Status         `json:"status,omitempty"`
-	Priority           int            `json:"priority,omitempty"`
+	Priority           int            `json:"priority"` // No omitempty: 0 is valid (P0/critical)
 	IssueType          IssueType      `json:"issue_type,omitempty"`
 	Assignee           string         `json:"assignee,omitempty"`
 	EstimatedMinutes   *int           `json:"estimated_minutes,omitempty"`