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>

client/mcproxy/client_test.go

@@ -32,7 +32,7 @@ func setupTestClient(t *testing.T) *Client {
 	if err != nil {
 		t.Fatalf("open db: %v", err)
 	}
-	t.Cleanup(func() { store.Close() })
+	t.Cleanup(func() { _ = store.Close() })
 
 	if err := store.Migrate(); err != nil {
 		t.Fatalf("migrate: %v", err)
@@ -128,7 +128,7 @@ func setupTestClient(t *testing.T) *Client {
 	if err != nil {
 		t.Fatalf("dial bufconn: %v", err)
 	}
-	t.Cleanup(func() { conn.Close() })
+	t.Cleanup(func() { _ = conn.Close() })
 
 	return &Client{
 		conn:   conn,