fix: Post-PR #8 critical improvements (bd-64, bd-65, bd-66, bd-67)

This commit addresses all critical follow-up issues identified in the
code review of PR #8 (atomic counter implementation).

## bd-64: Fix SyncAllCounters performance bottleneck (P0)
- Replace SyncAllCounters() on every CreateIssue with lazy initialization
- Add ensureCounterInitialized() that only scans prefix-specific issues on first use
- Performance improvement: O(n) full table scan → O(1) for subsequent creates
- Add comprehensive tests in lazy_init_test.go

## bd-65: Add migration for issue_counters table (P1)
- Add migrateIssueCountersTable() similar to migrateDirtyIssuesTable()
- Checks if table is empty and syncs from existing issues on first open
- Handles both fresh databases and migrations from old databases
- Add comprehensive tests in migration_test.go (3 scenarios)

## bd-66: Make import counter sync failure fatal (P1)
- Change SyncAllCounters() failure from warning to fatal error in import
- Prevents ID collisions when counter sync fails
- Data integrity > convenience

## bd-67: Update test comments (P2)
- Update TestMultiProcessIDGeneration comments to reflect fix is in place
- Change "With the bug, we expect errors" → "After the fix, all should succeed"

All tests pass. Atomic counter implementation is now production-ready.

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

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

cmd/bd/import.go

@@ -240,9 +240,11 @@ Behavior:
 
 		// Phase 5: Sync ID counters after importing issues with explicit IDs
 		// This prevents ID collisions with subsequently auto-generated issues
+		// CRITICAL: If this fails, subsequent auto-generated IDs WILL collide with imported issues
 		if err := sqliteStore.SyncAllCounters(ctx); err != nil {
-			fmt.Fprintf(os.Stderr, "Warning: failed to sync ID counters: %v\n", err)
-			// Don't exit - this is not fatal, just a warning
+			fmt.Fprintf(os.Stderr, "Error: failed to sync ID counters: %v\n", err)
+			fmt.Fprintf(os.Stderr, "Cannot proceed - auto-generated IDs would collide with imported issues.\n")
+			os.Exit(1)
 		}
 
 		// Phase 6: Process dependencies