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/version.go

@@ -55,7 +55,7 @@ func showDaemonVersion() {
 		fmt.Fprintf(os.Stderr, "Hint: start daemon with 'bd daemon'\n")
 		os.Exit(1)
 	}
-	defer client.Close()
+	defer func() { _ = client.Close() }()
 
 	health, err := client.Health()
 	if err != nil {