Fix golangci-lint v2 compliance, make all passes clean

- Fix 314 errcheck violations (blank identifier for unrecoverable errors)
- Fix errorlint violation (errors.Is for io.EOF)
- Remove unused serveL7Route test helper
- Simplify Duration.Seconds() selectors in tests
- Remove unnecessary fmt.Sprintf in test
- Migrate exclusion rules from issues.exclusions to linters.exclusions (v2 schema)
- Add gosec test exclusions (G115, G304, G402, G705)
- Disable fieldalignment govet analyzer (optimization, not correctness)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/proxy/proxy.go

@@ -2,6 +2,7 @@ package proxy
 
 import (
 	"context"
+	"errors"
 	"io"
 	"net"
 	"sync"
@@ -31,8 +32,8 @@ func Relay(ctx context.Context, client, backend net.Conn, peeked []byte, idleTim
 
 	go func() {
 		<-ctx.Done()
-		client.Close()
-		backend.Close()
+		_ = client.Close()
+		_ = backend.Close()
 	}()
 
 	var (
@@ -50,7 +51,7 @@ func Relay(ctx context.Context, client, backend net.Conn, peeked []byte, idleTim
 		result.ClientBytes, errC2B = copyWithIdleTimeout(backend, client, idleTimeout)
 		// Half-close backend's write side.
 		if hc, ok := backend.(interface{ CloseWrite() error }); ok {
-			hc.CloseWrite()
+			_ = hc.CloseWrite()
 		}
 	}()
 
@@ -60,7 +61,7 @@ func Relay(ctx context.Context, client, backend net.Conn, peeked []byte, idleTim
 		result.BackendBytes, errB2C = copyWithIdleTimeout(client, backend, idleTimeout)
 		// Half-close client's write side.
 		if hc, ok := client.(interface{ CloseWrite() error }); ok {
-			hc.CloseWrite()
+			_ = hc.CloseWrite()
 		}
 	}()
 
@@ -85,10 +86,10 @@ func copyWithIdleTimeout(dst, src net.Conn, idleTimeout time.Duration) (int64, e
 	var total int64
 
 	for {
-		src.SetReadDeadline(time.Now().Add(idleTimeout))
+		_ = src.SetReadDeadline(time.Now().Add(idleTimeout))
 		nr, readErr := src.Read(buf)
 		if nr > 0 {
-			dst.SetWriteDeadline(time.Now().Add(idleTimeout))
+			_ = dst.SetWriteDeadline(time.Now().Add(idleTimeout))
 			nw, writeErr := dst.Write(buf[:nr])
 			total += int64(nw)
 			if writeErr != nil {
@@ -96,7 +97,7 @@ func copyWithIdleTimeout(dst, src net.Conn, idleTimeout time.Duration) (int64, e
 			}
 		}
 		if readErr != nil {
-			if readErr == io.EOF {
+			if errors.Is(readErr, io.EOF) {
 				return total, nil
 			}
 			return total, readErr