refactor(lint): apply Go best practices across codebase

- Use errors.Is() instead of direct sentinel comparison (13 instances)
- Convert if/else-if chains to switch/case for same-variable comparisons
- Remove redundant bitwise OR with zero
- Add post-implementation checklist to CLAUDE.md

cmd/gateway_providers.go

@@ -170,21 +170,22 @@ func registerProvidersFromDB(registry *providers.Registry, provStore store.Provi
 		if p.APIKey == "" {
 			continue
 		}
-		if p.ProviderType == store.ProviderChatGPTOAuth {
+		switch p.ProviderType {
+		case store.ProviderChatGPTOAuth:
 			ts := oauth.NewDBTokenSource(provStore, secretStore, p.Name)
 			registry.Register(providers.NewCodexProvider(p.Name, ts, p.APIBase, ""))
-		} else if p.ProviderType == store.ProviderAnthropicNative {
+		case store.ProviderAnthropicNative:
 			registry.Register(providers.NewAnthropicProvider(p.APIKey,
 				providers.WithAnthropicBaseURL(p.APIBase)))
-		} else if p.ProviderType == store.ProviderDashScope {
+		case store.ProviderDashScope:
 			registry.Register(providers.NewDashScopeProvider(p.APIKey, p.APIBase, ""))
-		} else if p.ProviderType == store.ProviderBailian {
+		case store.ProviderBailian:
 			base := p.APIBase
 			if base == "" {
 				base = "https://coding-intl.dashscope.aliyuncs.com/v1"
 			}
 			registry.Register(providers.NewOpenAIProvider(p.Name, p.APIKey, base, "qwen3.5-plus"))
-		} else {
+		default:
 			prov := providers.NewOpenAIProvider(p.Name, p.APIKey, p.APIBase, "")
 			if p.ProviderType == store.ProviderMiniMax {
 				prov.WithChatPath("/text/chatcompletion_v2")

cmd/gateway_tsnet.go

@@ -4,6 +4,7 @@ package cmd
 
 import (
 	"context"
+	"errors"
 	"log/slog"
 	"net"
 	"net/http"
@@ -59,7 +60,7 @@ func initTailscale(ctx context.Context, cfg *config.Config, mux http.Handler) fu
 
 	httpSrv := &http.Server{Handler: mux}
 	go func() {
-		if err := httpSrv.Serve(ln); err != nil && err != http.ErrServerClosed {
+		if err := httpSrv.Serve(ln); err != nil && !errors.Is(err, http.ErrServerClosed) {
 			slog.Warn("Tailscale HTTP server error", "error", err)
 		}
 	}()

cmd/migrate.go

@@ -3,6 +3,7 @@ package cmd
 import (
 	"context"
 	"database/sql"
+	"errors"
 	"fmt"
 	"log/slog"
 	"os"
@@ -93,7 +94,7 @@ func migrateUpCmd() *cobra.Command {
 			}
 			defer m.Close()
 
-			if err := m.Up(); err != nil && err != migrate.ErrNoChange {
+			if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
 				return fmt.Errorf("migrate up: %w", err)
 			}
 
@@ -139,7 +140,7 @@ func migrateDownCmd() *cobra.Command {
 			if steps <= 0 {
 				steps = 1
 			}
-			if err := m.Steps(-steps); err != nil && err != migrate.ErrNoChange {
+			if err := m.Steps(-steps); err != nil && !errors.Is(err, migrate.ErrNoChange) {
 				return fmt.Errorf("migrate down: %w", err)
 			}
 
@@ -226,7 +227,7 @@ func migrateGotoCmd() *cobra.Command {
 			}
 			defer m.Close()
 
-			if err := m.Migrate(uint(version)); err != nil && err != migrate.ErrNoChange {
+			if err := m.Migrate(uint(version)); err != nil && !errors.Is(err, migrate.ErrNoChange) {
 				return fmt.Errorf("migrate goto: %w", err)
 			}
 			slog.Info("migrated to version", "version", version)

cmd/upgrade.go

@@ -168,7 +168,7 @@ func runUpgrade(dryRun bool) error {
 		}
 		defer m.Close()
 
-		if err := m.Up(); err != nil && err != migrate.ErrNoChange {
+		if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
 			fmt.Println("FAILED")
 			return fmt.Errorf("migrate up: %w", err)
 		}
@@ -240,7 +240,7 @@ func checkSchemaOrAutoUpgrade(dsn string) error {
 		}
 		defer m.Close()
 
-		if mErr := m.Up(); mErr != nil && mErr != migrate.ErrNoChange {
+		if mErr := m.Up(); mErr != nil && !errors.Is(mErr, migrate.ErrNoChange) {
 			return fmt.Errorf("auto-upgrade: migrate up: %w", mErr)
 		}