Fix bd-346: Auto-flush after renumber/rename-prefix now does full export

Problem: Incremental flush merged dirty issues with existing JSONL, leaving
old IDs when issues were renamed (e.g., test-3 remained after renumbering to test-2).

Solution:
- Add needsFullExport flag to force complete JSONL rebuild from DB
- Skip loading existing JSONL when fullExport=true (start with empty map)
- Use markDirtyAndScheduleFullExport() in renumber and rename-prefix commands
- PersistentPostRun flushes immediately before process exits (respects fullExport)

Test: Verified renumber with gaps correctly exports only current IDs to JSONL

cmd/bd/renumber.go

@@ -128,8 +128,8 @@ Risks:
 			os.Exit(1)
 		}
 
-		// Schedule auto-flush to export changes
-		markDirtyAndScheduleFlush()
+		// Schedule full export (IDs changed, incremental won't work)
+		markDirtyAndScheduleFullExport()
 
 		fmt.Printf("%s Successfully renumbered %d issues\n", green("✓"), len(issues))