Enable errcheck linter and fix all production code warnings

- Enabled errcheck linter (previously disabled)
- Set tests: false in .golangci.yml to focus on production code
- Fixed 27 errcheck warnings using Go best practices:
  * Database resources: defer func() { _ = rows.Close() }()
  * Transaction rollbacks: defer func() { _ = tx.Rollback() }()
  * Best-effort closers: _ = store.Close(), _ = client.Close()
  * File writes: proper error checking on Close()
  * Interactive input: handle EOF gracefully
  * File ops: ignore ENOENT on os.Remove()
- All tests pass
- Closes bd-58

Amp-Thread-ID: https://ampcode.com/threads/T-57c9afd3-9adf-40c2-8be7-3e493d200361
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/renumber.go

@@ -55,13 +55,13 @@ Risks:
 			}
 			store, err = sqlite.New(dbPath)
 			if err != nil {
-				fmt.Fprintf(os.Stderr, "Error: failed to open database: %v\n", err)
-				os.Exit(1)
+			fmt.Fprintf(os.Stderr, "Error: failed to open database: %v\n", err)
+			os.Exit(1)
+			}
+			defer func() { _ = store.Close() }()
 			}
-			defer store.Close()
-		}
 
-		ctx := context.Background()
+			ctx := context.Background()
 
 		// Get prefix from config, or derive from first issue if not set
 		prefix, err := store.GetConfig(ctx, "issue_prefix")