style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

tools/openapigen/walker.go

@@ -57,9 +57,7 @@ func walkPackages(requests []packageRequest) ([]Schema, []Alias, error) {
 							}
 							overrides := req.Overrides[ts.Name.Name]
 							for _, fld := range strct.Fields.List {
-								for _, f := range buildFields(fld, overrides) {
-									s.Fields = append(s.Fields, f)
-								}
+								s.Fields = append(s.Fields, buildFields(fld, overrides)...)
 							}
 							schemas = append(schemas, s)
 							continue