Fix bd-c6cf: Force full export when export_hashes is cleared

When validateJSONLIntegrity() clears export_hashes due to hash mismatch
or missing JSONL, the subsequent export now correctly exports ALL issues
instead of only dirty ones, preventing permanent database divergence.

Changes:
- validateJSONLIntegrity() returns (needsFullExport, error) to signal when
  export_hashes was cleared
- flushToJSONL() moved integrity check BEFORE isDirty gate so integrity
  issues trigger export even when nothing is dirty
- Missing JSONL treated as non-fatal force-full-export case
- Increased scanner buffer from 64KB to 2MB to handle large JSON lines
- Added scanner.Err() check to catch buffer overflow errors
- Updated all tests to verify needsFullExport flag

Fixes database divergence issue where clearing export_hashes didn't
trigger re-export, causing 5 issues to disappear from JSONL in fred clone.

Amp-Thread-ID: https://ampcode.com/threads/T-bf2fdcd6-7bbd-4c30-b1db-746b928c93b8
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/jsonl_integrity_test.go

@@ -85,9 +85,13 @@ func TestJSONLIntegrityValidation(t *testing.T) {
 	
 	// Test 1: Validate with matching hash (should succeed)
 	t.Run("MatchingHash", func(t *testing.T) {
-		if err := validateJSONLIntegrity(ctx, jsonlPath); err != nil {
+		needsFullExport, err := validateJSONLIntegrity(ctx, jsonlPath)
+		if err != nil {
 			t.Fatalf("validation failed with matching hash: %v", err)
 		}
+		if needsFullExport {
+			t.Fatalf("expected needsFullExport=false for matching hash")
+		}
 	})
 	
 	// Test 2: Modify JSONL file (simulating git pull) and validate
@@ -103,9 +107,13 @@ func TestJSONLIntegrityValidation(t *testing.T) {
 		}
 		
 		// Validate should detect mismatch and clear export_hashes
-		if err := validateJSONLIntegrity(ctx, jsonlPath); err != nil {
+		needsFullExport, err := validateJSONLIntegrity(ctx, jsonlPath)
+		if err != nil {
 			t.Fatalf("validation failed: %v", err)
 		}
+		if !needsFullExport {
+			t.Fatalf("expected needsFullExport=true after clearing export_hashes")
+		}
 		
 		// Verify export_hashes were cleared
 		hash, err := testStore.GetExportHash(ctx, "bd-1")
@@ -135,9 +143,13 @@ func TestJSONLIntegrityValidation(t *testing.T) {
 		}
 		
 		// Validate should detect missing file and clear export_hashes
-		if err := validateJSONLIntegrity(ctx, jsonlPath); err != nil {
+		needsFullExport, err := validateJSONLIntegrity(ctx, jsonlPath)
+		if err != nil {
 			t.Fatalf("validation failed: %v", err)
 		}
+		if !needsFullExport {
+			t.Fatalf("expected needsFullExport=true after clearing export_hashes")
+		}
 		
 		// Verify export_hashes were cleared
 		hash, err := testStore.GetExportHash(ctx, "bd-1")